Lisp is kinda hot again, Perl 6 will have multiple dispatch, Ruby's getting before and after methods, and Aspect Oriented Programming is still bubbling under in Java, so what better time to revisit the Common Lisp Object System and this slim volume from 1989? Unsurprisingly, objects in Lisp are implemented differently to the well-known object systems, even Smalltalk, so if you're into OO (and in particular AOP), check out CLOS.
And this book is an excellent way to get acquainted with the features of CLOS. While the introduction suggests non-Lispniks can benefit from reading this book, I would strongly suggest you can read Lisp to make sure you understand what's going on. It's a quick read (about 220 pages, excluding the appendices), the language is unfussy and clear, and the concepts are simply explained.
The hoary old shape example is used briefly (perhaps it wasn't as tired in 1989), but the two main examples used in the book is a simple locking mechanism for concurrency, and multiple inheritance based disk and tape stream classes. Both example do the job well of highlighting the unique flavour of CLOS. The metaobject protocol is touched upon only briefly, but there's more than enough of the standard object protocol described for you to be getting your money's worth.
Other introductory books do cover CLOS (Practical Common Lisp has a good chapter), but this is a very nice standalone volume devoted to CLOS only. Well worth reading, object (and Lisp) fans.