PCL: (Practical Common Lisp): http://www.gigamonkeys.com/book/ (chapters 16 & 17)
Keene: (Object-Oriented Programming in COMMON LISP: A Programmer's Guide to CLOS): http://www.amazon.com/Object-Oriented-Programming-COMMON-LIS...
Warp Speed intro: http://xach.com/lisp/jrm-clos-guide.html