This is the Lisp programmers' bible. If you need to know the official specification, every function defined in Common Lisp can be found in here somewhere. Anyone vaguely serious about programming in Lisp for extended periods of time should keep a copy of this book for reference. However, note that while this book is clearly written, it is not an introduction to programming in Lisp--Common Lisp: The Language only offers the language specification.
Guy Steele has a talent for making something like a programming language spec interesting and easy to read. The information in this book is freely available online, but I collect older technical books just to keep around for the times I want to flip through them. The paperback is short and thick, but is pretty good quality, and worth looking for if you're into books of this nature.