What can I say that others haven't said about Peter Seibel's new Practical Common Lisp?
It is now the best written introduction-to-intermediate Common Lisp book you can buy. It's great reading, the practical focus of the examples are well chosen and meaningful, and he jumps right in with the power of Lisp, instead of doing the pedagogically boring "this is a list" and "this is cons, car, and cdr".
I especially like the domain-specific language approach the book uses. In particular, Chapter 24 - Practical: Parsing Binary Files, and the later chapters are excellent examples of what you can really do with macros. In fact, I think Chapter 24 is standalone to point that, with some warning, you could show it to someone interested in the kind of power you get with macros, but doesn't want to wade through the whole book to get to the punchline. That person wouldn't understand a lot of the code, but would certainly see the ability of creating macros that can write a significant amount of straightforward boilerplate code.
I can't wait to get my copy.