This is a nice "tutorial style" walk through much of the functionality of CLOS.
It doesn't get into examples that are of _spectacular_ complexity, either in size or in difficulty of concept, but that strikes me as being just fine. The dialect of CLOS in use predates the final version that was released, so a few examples require a little modification to work with modern Lisp releases.
If you're looking to get into the sophistication of redefining your own metaobject protocols, the MOP book is surely more suitable. But for most of us who just need to figure out what CLOS is good for, this book provides a very nice presentation of that.