I'm trying to reproduce a title from an older book, which uses what it seems to be a lowercase numero sign (see image below). Is there a symbol like this in latex (or even unicode for that matter!)?
Currently I simply doing this: n.\textsuperscript{o} where the "o" is the lowercase letter "O".
From your question I take, that unicode is favorable to you. In that case, you might want to use the xetex or luatex engines which natively support unicode input.
Unicode has only one codepoint (u+2116) for the numero sign, which should only be used for cyrillic and asian, according to wikipedia, and incorporates a capital N.
According to the same source, it's Italian practice to write Numero as n+superscript lowercase underlined o, which can be replaced by "n." or n+Ordinal symbol , which is part of unicode as well: u+00AB.
Your source seems to mix these two replacement versions.
The following code shows the different possibilities (Numero sign, n.+Ordinal sign, n.+superscript o, n.+degree symbol, which would be the xetex equivalent of the \circ proposed in the comments) using the fonts TeX Gyre Pagella and STIXGeneral via xelatex:
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\setmainfont{TeX Gyre Pagella}
№ n.\kern-0.5bpº n.\kern-0.5bp\textsuperscript{o} n.\kern-0.5bp°
\setmainfont{STIXGeneral}
№ n.\kern-0.5bpº n.\kern-0.5bp\textsuperscript{o} n.\kern-0.5bp°
\end{document}
I added the kerning, because the o/ordinal symbol in the original source seems to be horizontally closer to the . than in the produced copy.
The result shows, that STIXGeneral uses an Ordinal sign without underlining, which seems to be the most appropriate to me. But this depends on the font as TeX Gyre Pagella uses an underlined Ordinal sign.
So n.+superscript o seems to be the most practical way to go.
n.$^\circ$– Steven B. Segletes Mar 10 '13 at 3:20\textordmasculine? Although that is not what appears to be in the original. – jja Mar 10 '13 at 3:29\circ, like Steven suggested. But I had to do something similar recently and I went with the one I mentioned, so I thought you might like to hear about it as well, as a possibility. – jja Mar 10 '13 at 5:38