The default value of the oddsidemargin

Question

When I use the grid behind the text I see on my first page the distance between the left side of the page and the left margin of the text is around 1.5 inch.

When I use the layout package and the \layout command I see the distance between the left side of the page and the left margin of the text is \oddsidemargin+ one inch+ \hoffset=35pt+1 inch which is around the same measured value 1.5 inch.

So far everything looks fine. But I am surprised why the default value of the oddsidemargin is 18.06749pt on the page 13 of the Layout package instead of 35pt we see as a result of \layout command below.

The other surprising point is in the \LaTeXe: An Unofficial Reference Manual, page 26, the oddsidemargin is 40% of the difference between the paper width (8.5 inch) and the textwidth (around 4.5 inch) which is 1.6 inch and not 35 pt in the layout package. Below is my code:

\documentclass{book}
\usepackage[grid,
 gridcolor=red!50,
 subgridcolor=green!50,
gridunit=in]{eso-pic}
\usepackage{layout}

\begin{document}

\chapter{Oddsidemargin}
When I use the grid behind the text I see on my first page the distance between the left side of the 
page and the left margin of the text is around 1.5 inch.

When I use the layout package and the \textbackslash layout command I see the distance between the 
left side of the page and the left margin of the text is \textbackslash oddsidemargin+ one inch+ 
\textbackslash hoffset=35pt+1 inch which is around the same measured value 1.5 inch.

So far everything looks fine. But I am surprised why the default value of the oddsidemargin is 
18.06749pt on the page 13 of the Layout package instead of 35pt we see as a result of  \textbackslash 
layout command below.

The other surprising point is in the \LaTeXe: An Unofficial Reference Manual, page 26, the 
oddsidemargin is 40\% of the difference between the paper width (8.5 inch) and the textwidth (around 
4.5 inch) which is 1.6 inch and not 35 pt in the layout package. 

\layout


\end{document}

Answer 1

You can ask LaTeX to show you the values.

\typeout{  start: \the\oddsidemargin}
\documentclass{book}

\typeout{preamble: \the\oddsidemargin}

\begin{document}

\typeout{  page 1: \the\oddsidemargin}

z
\end{document}

produces

   start: 0.0pt
preamble: 35.0pt
  page 1: 35.0pt

So the default value in the format is 0pt, but as you are using book class with the default usletter, 10pt, onecolumn and twoside options it gets set to 35pt

You can see the setting in size10.clo

It gets set to .4(\paperwidth-\textwidth)-1in = 0.4(614.295pt-345pt)-72.27pt ~ 35.4pt

    \setlength\@tempdima        {\paperwidth}
    \addtolength\@tempdima      {-\textwidth}
    \setlength\oddsidemargin    {.4\@tempdima}
    \addtolength\oddsidemargin  {-1in}

Then rounded to a whole number of pt = 35pt

  \@settopoint\oddsidemargin

The actual margin is \oddsidemargin+1in so 1+35/72.27 in ~ 1.48in

All of the above is at the default US Letter paper size, if you add [a4paper] then \paperwidth is different and the final value ends up being 28pt rather than 35pt.