<p>I've recently had a need to do a bit of lisp editing and I found the nifty Ready Lisp package for OS X, which is great, except Aquamacs automatically uses a proportional font (which is idiotic, IMHO) and I want to change it to a monospace font. However, I'm not really much of an EMACS user, and the preferences menu in Aquamacs is less than clear on where and how one might make such a change. </p>

From the
EmacsWiki Aquamacs FAQ
:
To change the font used to display the
current frame, go to the font panel.
You can do this with the keystroke
Apple-t, or via the menu: Options →
Show/Hide → Font Panel. Once there,
select the font you want.
To make the current frame’s font the
default, go to Options → Frame
Appearance Styles. Select “use current
style for foo mode”, where foo is the
mode of the current frame (e.g.,
foo=text for text mode), to use the
current style (including the font, but
also any other changes you’ve made to
the frame’s style) for all files of
this type. Select “use current style
as default” to use the current style
for all files for whose major mode no
special style has been defined.
There are also recommendations for monospaced fonts - Monaco or "Vera Sans Mono".