Warning: long answer ahead!
The setspace package has a long history that dates back
(perhaps with a different name, I don’t remember) to the late
Eighties of the past century, to the ancient days of LaTeX 2.09
and TeX2; the comments at the beginning of setspace.sty witness
to this, as well as to a long and not always linear incremental
development, during which provisional solutions were being added
to solve problems and bugs as they were reported. The comments
also warn you that, even in the latest release (my version,
distributed with TeX Live 2015, is copyrighted 2011), some issues
may remain unresolved; in particular, lines 284–285 read
\begin{singlespace}\begin{quote} produces the wrong spacing
before the quote (extra glue is inserted).
Indeed, the anomalous behavior complained about in the question
comes, IMHO, out of a tiny bug in the \singlespacing command;
I will try to analyze it in detail in this answer, and to
propose a correction.
Detailed analysis
Quoting again from the comments of setspace.sty, lines 54–58:
The extra \vskip in the definition of \singlespacing seems
to make for a cleaner transition from multiple spacing back to
single spacing. These did not appear warrranted [sic] for
other size changes.
What is this about? Well, multiple spacing is achieved,
obviously, by incrementing the leading between the lines, that is
(Oxford Dictionary of English, as presented by the Dictionary
application of Mac OS X), “the distance from the bottom of one
line of type to the bottom of the next”. Now, (La)TeX applies
the same leading not only to all lines of a given paragraph,
but also between the first line of that paragraph and the last
line of the previous one. This means that, when two successive
paragraphs are typeset with different leadings, the leading
between the last line of the first and the first line of the
second will always be the same as the leading between the lines
of the second paragraph. When you change from a smaller leading
to a larger one, this looks right, but it does not the other way
around (just think of it): in the latter case, it rather seems
that the larger leading should be used between the two paragraphs
too.
In an attempt to make up for this, the \singlespacing command
adds an extra blank line, that has the new (single-line) leading
both above and below itself:
\newcommand{\singlespacing}{%
\setstretch {\setspace@singlespace}% normally 1
\vskip \baselineskip % Correction for coming into singlespace
}
But this would yield the desired effect only if the “old”
leading were exactly twice as tall as the “new” one; so:
it evidently does not work when changing from
\onehalfspacing to \singlespacing, as it is perhaps
hinted at by the comment on lines 54–58;
it doesn’t work even when changing from \doublespacing to
\singlespacing, because the leading of the former style is
not at all twice as much as the leading of the latter.
Indeed, what is normally considered “double spacing” is a spacing
in which the baselines of text are separated by an amount of
vertical space that is twice as much as the font size; for
example, for a font size of 11 points, as it is in the question,
the baselines should be 22 points apart. But the leading in
\singlespacing conditions is already set to 120%—125% of the
font size, e.g., to 13.6 points for a size of 11 points:
setting the leading to twice as much would result in a leading of
27.2 point, that is, in almost “two-lines-and-a-half spacing”.
For this reason, the setspace package implements \doublespace
by setting the multiplication factor for the leading to values
actually smaller than two:
\newcommand{\doublespacing}{%
\setstretch {1.667}% default
\ifcase \@ptsize \relax % 10pt
\setstretch {1.667}%
\or % 11pt
\setstretch {1.618}%
\or % 12pt
\setstretch {1.655}%
\fi
}
As you see, for 11pt a multiplier of 1.618 is used (reminiscent
of the golden ratio?), yielding an effective leading of 1.618 *
13.6pt = 22,0048pt, that is, almost exactly the double of the
font size, as intended. In conclusion, the \vskip \baselineskip
issued by the \singlespacing command turns out to be overcompensation,
and this is precisely the source of the extra space that has been asked
about.
We can easily confirm this by compiling a slightly modified
version of the OP’s MWE:
\documentclass[11pt,oneside]{book}
\usepackage[letterpaper]{geometry}
\usepackage{setspace}
\expandafter\def\expandafter\quote\expandafter{\quote\singlespacing}
\expandafter\def\expandafter\quotation\expandafter{\quotation\singlespacing}
\doublespacing
\parskip=0pt
\usepackage{lipsum}
\begin{document}
\lipsum[1]
\begin{quotation}
Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget,
consectetuer id, vulputate a, magna. Donec vehicula augue eu neque.
Pellentesque habitant morbi tristique senectus et netus et malesuada fames
ac turpis egestas.
\end{quotation}
\lipsum[2]
\singlespacing
\lipsum[3]
\showboxbreadth = 1000
\showboxdepth = 10
\showlists
\end{document}
This code produces the following output:
The tracing commands included near the end of the code write into
the transcript file (among other things) detailed information
about the space that goes between the lines. The beginning of
this diagnostic tracing reads as follows:
### vertical mode entered at line 0
### current page:
\write-{}
\glue(\topskip) 3.39584
\hbox(7.60416+2.12917)x430.00462, glue set - 0.16643
.\hbox(0.0+0.0)x17.0
.\OT1/cmr/m/n/10.95 L
.\OT1/cmr/m/n/10.95 o
.\OT1/cmr/m/n/10.95 r
.\OT1/cmr/m/n/10.95 e
.\OT1/cmr/m/n/10.95 m
.\glue 3.65 plus 1.825 minus 1.21666
…96 lines omitted…
.\glue(\rightskip) 0.0
\penalty 150
\glue(\baselineskip) 12.27142
\hbox(7.60416+2.12917)x430.00462, glue set 0.52057
.\OT1/cmr/m/n/10.95 p
.\OT1/cmr/m/n/10.95 l
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 c
The \hbox that begins on the fifth line is the first typeset
line of the first paragraph, which ends with .\glue(\rightskip) 0.0
about a hundred lines below; we can see that the depth of
this line is 2.12917 (points). A \penalty 150 follows, coming
from \clubpenalty, and then an “interline glue” of 12.27142
points (\glue(\baselineskip) 12.27142), which is precisely the
vertical space that TeX inserts between two typeset lines to
ensure that their respective baseline are as much apart as
desired. Since the next typeset line, that begins with
\hbox(7.60416+2.12917)x430.00462..., has a height of 7.60416
points, we see that TeX is assuming a leading of 2.12917 +
12.27142 + 7.60416 = 22,00475 points, as claimed above (within
rounding errors.
Let‘s now have a look to the excerpt that corresponds to the
beginning of the quotation environment:
\hbox(7.60416+2.12917)x430.00462, glue set 115.40472fil
.\OT1/cmr/m/n/10.95 s
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 g
.\OT1/cmr/m/n/10.95 i
.\OT1/cmr/m/n/10.95 t
…60 lines omitted…
.\glue 3.65 plus 1.825 minus 1.21666
.\OT1/cmr/m/n/10.95 r
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 t
.\OT1/cmr/m/n/10.95 r
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 m
.\OT1/cmr/m/n/10.95 .
.\penalty 10000
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0
\penalty -51
\glue 12.0 plus 4.0 minus 6.0
\glue 13.6
\glue(\parskip) 0.0 plus 1.0
\glue(\baselineskip) 3.86668
\hbox(7.60416+2.12917)x375.2545, glue set 0.4625, shifted 27.37506
The \hbox whose contents begin with the five characters
“s-a-g-i-t” and ends with “r-u-t-r-u-m” + other stuff +
.\glue(\rightskip) 0.0 is the last typeset line of the first
paragraph, immediately above the quotation; we report this line
too, because we need to know that its depth is (again) 2.12917
points. After this box we find:
\penalty -51 (bonus for breaking the page before the
quotation);
\glue 12.0 plus 4.0 minus 6.0 (vertical space inserted
above quotations);
\glue 13.6, which is the (over-)compensation inserted by
the \singlespacing command for the change in the spacing;
\glue(\parskip) 0.0 plus 1.0 (vertical space inserted
between paragraphs);
\glue(\baselineskip) 3.86668 (see below);
\hbox(7.60416+2.12917)x375.2545..., which is the beginning
of the box that contains the first typeset line of the
quotation; note that its height is 7.60416 (points).
If we add to this height the depth of the previous line and the
interline glue, we can check that the latter has been computed to
attain a leading of 7.60416 + 2.12917 + 3.86668 = 13.60001
(always points), that is, within rounding errors, the “normal”
(for an 11-point font) leading of 13.6 points. The overall
distance between the baseline of the last line of the preceding
paragraph and that of the first line of the quotation is 13.60001
+ 13.6 + 12.0 ≈ 39.2 points (omitting stretch and shrink
components); but it had better been 22.0048 + 12.0 ≈ 34.0 points,
that is [“double-spacing” leading] + [space above quotation].
Note that the difference amounts to about 1.83mm, that is, enough
to be clearly perceived.
To confirm that the distance should have been as much as just
claimed, consider the transcript of what happens at the end of
the quotation environment:
\hbox(7.60416+2.12917)x375.2545, glue set - 0.66791, shifted 27.37506
.\OT1/cmr/m/n/10.95 h
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 b
.\OT1/cmr/m/n/10.95 i
…85 lines omitted…
.\OT1/cmr/m/n/10.95 t
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 s
.\OT1/cmr/m/n/10.95 .
.\penalty 10000
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0
\penalty -51
\glue 12.0 plus 4.0 minus 6.0
\glue(\parskip) 0.0
\glue(\baselineskip) 12.27142
\hbox(7.60416+2.12917)x430.00462, glue set - 0.10161
.\OT1/cmr/m/n/10.95 N
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 m
.\glue 3.65 plus 1.825 minus 1.21666
.\OT1/cmr/m/n/10.95 d
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 i
.\glue 3.65 plus 1.825 minus 1.21666
.\OT1/cmr/m/n/10.95 l
.\OT1/cmr/m/n/10.95 i
.\OT1/cmr/m/n/10.95 g
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 l
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 ,
Once again, the transcript starts with the beginning of the last
line of the quotation (“h-a-b-i-…”), so that you can see that its
depth is (once again) 2.12917 points, and finishes reporting the
beginning of the first line of the paragraph that follows the
quotation (“N-a-m [space] d-u-i [space] l-i-g-u-l-a [comma]…”,
whose height is (this too) 7.60416 points. Analyzing this
transcript along the lines given above, we can check that the
\glue(\baselineskip) 12.27142 has been calculated for a leading
of 2.12917 + 12.27142 + 7.60416 = 22,00475 points, exactly as
between any two lines of a paragraph typeset while
\doublespacing is in force (see above). Thus, the total
distance between the baseline of the last line of the quotation
and that of the first line of the ensuing paragraph is 22.00475 +
12.0 ≈ 34.0 points (instead of the 39.2 points that we found
above the quotation).
You can similarly check that there is the same amount of excess
space (overcompensation) before the last paragraph, where just a
switch from \doublespacing to \singlespacing occurs, without
the complication of a quotation environment being entered. The
interline glue inserted before the first line of the last
paragraph gives rise to a leading of 13.00001 points, and there
is, an addition, the usual (buggy) vertical space of 13.6 points;
once again, the total is (approximately) 27.2 points, instead of
22.0048, as it should be to replicate the leading between the two
last lines of the preceding paragraph. Anyway, this excess space
is clearly visible in the printout.
Proposed correction
The correction we propose is a patch to the \singlespacing
command; after that, the usual quote and quotation
environment can be employed, giving the correct result. In the
following code, this patch to \singlespacing is exactly the
segment delimited by \makeatletter and \makeatother.
\documentclass[11pt,oneside]{book}
\usepackage[letterpaper]{geometry}
\usepackage{setspace}
\usepackage{lipsum}
\makeatletter
\renewcommand*\singlespacing{%
\par % ensure vertical mode
\null % add fake line with previous leading still in force
\setstretch {\setspace@singlespace}% change leading
\nobreak
\vskip -\baselineskip % compensate for the fake line we added, but with
% the new leading
\vskip \z@skip % tell "\addvspace" and "\addpenalty" _not_ to remove the
% above correction
}
\makeatother
% I’d go for different names, anyway; or follow advice given in other answers to
% patch up the standard environments.
\newenvironment*{JoeMquote}{%
\singlespacing
\quote
}{\endquote}
\newenvironment*{JoeMquotation}{%
\singlespacing
\quotation
}{\endquotation}
\doublespacing
\setlength{\parskip}{0pt}
\begin{document}
\lipsum[1]
\begin{JoeMquotation}
Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget,
consectetuer id, vulputate a, magna. Donec vehicula augue eu neque.
Pellentesque habitant morbi tristique senectus et netus et malesuada fames
ac turpis egestas.
\end{JoeMquotation}
\lipsum[2]
\singlespacing
\lipsum[3]
\showboxbreadth = 1000
\showboxdepth = 10
\showlists
\end{document}
Note that we prefer to define two new environments, rather than
changing the default ones. Indeed, this answer is about finding
the exact compensation that yields the same vertical space
above and below the quotation, not about how to incorporate this
correction into the standard environments: the other answers have
already taken care of explaining this.
The printout that this code produces is shown next: you can
immediately check, at first sight, that this time the spacing
looks correct.
The tracing written to the transcript file confirms that the
spacing is indeed correct to the scaled point. This is the
beginning of the quotation environment, including the last line
above it:
\hbox(7.60416+2.12917)x430.00462, glue set 115.40472fil
.\OT1/cmr/m/n/10.95 s
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 g
.\OT1/cmr/m/n/10.95 i
.\OT1/cmr/m/n/10.95 t
…60 lines omitted…
.\glue 3.65 plus 1.825 minus 1.21666
.\OT1/cmr/m/n/10.95 r
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 t
.\OT1/cmr/m/n/10.95 r
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 m
.\OT1/cmr/m/n/10.95 .
.\penalty 10000
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0
\glue(\baselineskip) 19.87558
\hbox(0.0+0.0)x0.0
\penalty 10000
\glue -13.6
\glue 0.0
\penalty -51
\glue 12.0 plus 4.0 minus 6.0
\glue(\parskip) 0.0 plus 1.0
\glue(\baselineskip) 5.99585
\hbox(7.60416+2.12917)x375.2545, glue set 0.4625, shifted 27.37506
The distance between the baseline of the last line of the
preceding paragraph and the baseline of the first line of the
quotation is 2.12917 + 19.87558 - 13.6 + 12.0 + 5.99585 + 7.60416
= 34.00476, which corresponds exactly, within rounding errors, to
the expected value of [“double-spacing” leading] + [space above
quotation] = 22.00475 + 12.0 = 34.00475. On the other hand, at
the end of the quotation we find
\hbox(7.60416+2.12917)x375.2545, glue set - 0.66791, shifted 27.37506
.\OT1/cmr/m/n/10.95 h
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 b
.\OT1/cmr/m/n/10.95 i
…85 lines omitted…
.\OT1/cmr/m/n/10.95 t
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 s
.\OT1/cmr/m/n/10.95 .
.\penalty 10000
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0
\penalty -51
\glue 12.0 plus 4.0 minus 6.0
\glue(\parskip) 0.0
\glue(\baselineskip) 12.27142
\hbox(7.60416+2.12917)x430.00462, glue set - 0.10161
(as usual, the beginning of the first line of the following
paragraph is shown too). Here the distance between the two
baselines is 2.12917 + 12.0 + 7.60416 + 12.27142 = 34.00475,
exactly the same (as always, within rounding errors to the
scaled point) as above the quotation.
The same (but without the \glue 12.0 plus 4.0 minus 6.0)
happens at the \singlespacing change before the last paragraph,
as you can check by yourself:
\hbox(7.31305+0.0)x430.00462, glue set 394.62999fil
.\OT1/cmr/m/n/10.95 m
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 r
.\OT1/cmr/m/n/10.95 i
.\OT1/cmr/m/n/10.95 s
.\OT1/cmr/m/n/10.95 .
.\penalty 10000
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0
\glue(\baselineskip) 22.00475
\hbox(0.0+0.0)x0.0
\penalty 10000
\glue -13.6
\glue 0.0
\glue(\parskip) 0.0
\glue(\baselineskip) 5.99585
\hbox(7.60416+2.12917)x430.00462, glue set 0.28633
.\hbox(0.0+0.0)x17.0
.\OT1/cmr/m/n/10.95 N
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 l
.\OT1/cmr/m/n/10.95 l
.\OT1/cmr/m/n/10.95 a
.\glue 3.65 plus 1.825 minus 1.21666
.\OT1/cmr/m/n/10.95 m
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 l
.\OT1/cmr/m/n/10.95 e
.\OT1/cmr/m/n/10.95 s
.\OT1/cmr/m/n/10.95 u
.\OT1/cmr/m/n/10.95 a
.\OT1/cmr/m/n/10.95 d
.\OT1/cmr/m/n/10.95 a
.\glue 3.65 plus 1.825 minus 1.21666
A final remark: the correction should, and can, be implemented at
a more general level, encompassing all possible cases of change
of spacing. Rather that appending stray boxes and glue to the
vertical list, a general algorithm should simply compute the
difference between the old leading and the new one (the old minus
the new) and, if this difference is positive, add a vertical
space of exactly that amount.
\vskip1into add an inch, use a minus to subtract. Maybe you are detecting something having to do with tex using baseline skips, rather than otherwise.