Further, I think it should be emphasized how much there is to CLOS. I've seen a number of CLOS tutorials that stop after showing how to do in CLOS what can be done in other OO languages, and don't really indicate how much more there is, let alone show how to do those things. Keene's book does go in to this. There are a few very minor differences between what she describes and what is in the final standard though.
I think the Keene book should be avoided by all means. I don't think it is a good introduction into OOP or CLOS. My recommendation would be Peter Seibels Practical Common Lisp.