An Introduction to Programming in Emacs Lisp 8th Edition

I like this book a lot; I think it will take me a long way into personalizing my Emacs. I've just started, and it will take a while to go through, but I like it already.

This is obsolete by many moons. There is a revised Third Edition, available from the Free Software Foundation; zero dollars for the PDF, but you can get a printed version. This edition is no longer useful for anyone.