There are many more excellent resources, most of them crosslinked in one of the learning section.
- The Common Lisp HyperSpec, by X3J13 committee. The Common Lisp Standard
- Common Lisp the Language, 2nd Edition, by Guy Steele
- Common Lisp Document Repository a collection of emergent and de-facto standards
- Common-Lisp.net, project hosting and gateway
- The Common Lisp Wiki
There are many other great books you can find links to. These are the most popularly linked introductory works.
- Practical Common Lisp, by Peter Seibel
- Common Lisp: A Gentle Introduction to Symbolic Computation (PDF), by David S. Touretzky
- Common Lisp: An Interactive Approach (PDF), by Stuart C. Shapiro
- Successful Lisp: How to Understand and Use Common Lisp, by David B. Lamkins
Following these are two more advanced books, but beginners shouldn't be afraid of them, they're quite accessible and "classics".
- On Lisp, by Paul Graham
- Structure and Interpretation of Computer Programs, by Abelson, Sussman, and Sussman
The last is actually on a related Lisp dialect, Scheme, but it's very helpful book, generally. If you find you prefer Scheme, it's an excellent start.
Quick-guides / cheat sheets for Common Lisp:
- The Common Lisp Cookbook by various contributors, includes quick help on often used commands or features. (legacy SF site)
- Common Lisp Quick Reference is a print-it-yourself booklet for having a quick reference handy.
- Simplified Common Lisp Reference gives a quick glance at often used commands, ideal for newcomers.