I learned with ANSI CL, then read PG's On Lisp (which I downloaded from somewhere and printed, I don't think it was available). ANSI CL is great, the chapter where he writes an OO framework is just amazing. And On Lisp blew my mind.

Seibel's Practical Common Lisp is a good book, worth reading, but quite lacking in the "practical" aspect, which tends to be implementation-dependent.