Lisp (and to a lesser degree, Scheme) fascinates me. Simple in the extreme and yet incredibly powerful, it is a thing of beauty. I learned Lisp by writing Lisp interpreters and compilers - usually to embed in other applications. As an embedded language Lisp excels - it is among the easiest languages to implement. Implementing it well, however, is an altogether different proposition.
By programming language standards, Lisp (1958) is an ancient tongue. It is older than COBOL by a year and older than C by more than a decade. And it is timeless.
Don't laugh but it's the coolest thing to build a tiny and reasonably fast Lisp bytecode VM (Pair, a current project of mine that I never get to, weighs in at a mere 28K!) and then shove it inside some application and make it do all sorts of things machines were never supposed to. Like thinking, for example :).
Lisp & Scheme
muSE - Embeddable Scheme dialect and more importantly, its associated blog.
Lambda The Ultimate Programming Languages Weblog
Some Favorite Lisp and Scheme Books
Paradigms of Artificial Intelligence Programming: Case Studies in Common Lisp
Lisp in Small Pieces - Great for building Lisp interpreters and compilers.
Structure and Interpretation of Computer Programs - Don't buy this classic though - it's free online.