I just finished Object-Oriented Programming in Common Lisp: A Programmer's Guide to CLOS, by Sonja E. Keene. Here are my thoughts:

1. The book is pretty expensive. It's fairly thin at just 266 pages, including the index, but the price is $39.99 USD. The amount of information you get for your money is pretty small. This may or may not bother you.
2. The book covers CLOS in good detail, showing how to use various features like generic methods, generic dispatch, class inheritance, and initialization.
3. The examples used in the book are fairly simplistic. They lack some of the meat that you'd expect in a book like this. There are two fundamental examples used throughout the book: a set of mutex locks, and a set of streams classes. I found the mutex locks to be particularly humorous since Common Lisp doesn't standardize threads. This isn't to say that the classes aren't useful as a learning tool, but they can't effectively be used in a standard CL implementation like SBCL, CMUCL, or CLISP without some amount of modification.
4. The book has a nice reference section toward the end that details all the CLOS syntax for various forms. I think that this is the most useful part of the book, long term. The book is thin enough to be kept at the ready on a shelf close to your keyboard and pulled out for this section alone.
5. The book is fairly old. The copyright date is 1989. No updates have taken place since that time. I don't think that CLOS has changed much, but I think the book is a bit dated. Some of the examples show a bit of history associated with the author's employer at the time, Symbolics.

In short, I found the book underwhelming. On a scale of 1 to 5, I'd give it a 3. There are other introductions to CLOS that seem to give you a reasonable introduction without the high price. If you want to understand some of the more advanced CLOS programming techniques, like :before, :after, or :around, or method combination types, then this book has something good to offer. If you just need a short introduction to the basics, I think you could check out resources on the web and save your money.