Before there was an ANSI standard for Common Lisp, there was "Common Lisp: the Language." This book, also known as CLtL1, described the state of the consensus on Common Lisp, circa 1984.
During the ANSI standardization process, Steele published a second edition (often called CLtL2). This book contains the complete contents of the first edition, plus material on CLOS, conditions, pretty printing and iteration facilities.
The book does not correspond exactly with the ANSI standard: some details are different; some things from the standard are missing; and some things from CLtL2 are not in the final ANSI standard. Programmers should therefore be wary of using it as a reference. Nonetheless, the book is easier to read than the formal ANSI specification, and has additional commentary and material. Recommended, but don't treat it as canon.
See also:
- http://www.cs.cmu.edu/afs/cs.cmu.edu/project/ai-repository/ai/html/cltl/cltl2.html
- Mirror of the previous link: http://www-prod-gif.supelec.fr/docs/cltl/cltl2.html
- Differences between CLTL2 and the ANSI standard
- Past discussion thread: CLtL2 copyright question
- CLtL2 is available in Debian as package cltl.
Debian package