Q: Anybody knows what is the font used for code snippets in 'On Lisp'?

In Adobe Reader, Go to File -> Properties and click on the "Fonts" tab. It should be one of those. Looks like a bunch of CMsomethingsomethings.

The best programming font I've found is Andale Mono, which is part of the "Core Fonts for the Web" http://en.wikipedia.org/wiki/Core_fonts_for_the_Web which is available for free. It's a conservative font in that it takes up less visual realestate than Courier, but I think because of that it's lighter and easier on the eyes.