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.
- Differences between CLTL2 and the ANSI standard
The http://www.cs.cmu.edu/afs/cs.cmu.edu/project/ai-repository/ai/html/cltl/cltl2.html link seems to be a little broken at the moment - missing links to some pages like "Table of Contents". Some of the mirror (like http://www-prod-gif.supelec.fr/docs/cltl/cltl2.html have those links working.