Necesary, but unfortunately not great
This book is necessary for people writing Common Lisp. It's the closest you can get to a good description of the official standard (except for the standard itself, of course).
Unfortunately, it's not perfect. The descriptions are a bit boring and sometimes slightly confusing; it's also a bit difficult to find what you need in the book.
But I'd absolutely recommend it to anyone serious about programming in Common Lisp.
