"On Lisp" is excellent but is an advanced text; I would advise against it as a first book. PG's Ansi Common Lisp is pretty good and I use it as a quick reference before I go to The Common Lisp Reference.
It's an excellent text to be sure, but I wouldn't call it advanced. PCL was the the first real Lisp text I read, and I found it to be a superb introduction to the subject.
Just a clarification. I was referring to PG "On Lisp" as advanced. Not PCL. "On Lisp" is mostly about complex uses of macros. Hence the name -- programming "On" lisp instead of programming in lisp