I decided to learn Lisp for some reason this weekend. I even managed to write a couple of little CGI scripts with it. I mostly learned from "Common Lisp: A gentle introduction to Symbolic Computation," which is a pretty good book. Lisp is a little different from the languages I'm used to, so it actually took me a little long than expected to become familiar with it. But at least I finally know what lambda notation is and how it works.
Another online book that my be useful to the Lisp beginner is "On Lisp". I haven't had a chance to read much of that one yet.
This page has links to lots more information about Lisp, including a link to the ANSI specification for Common Lisp and a couple of other references. This is the reference I've been looking for!