Many people unfamiliar with Common Lisp believe it's a purely
functional language, but this is untrue. While you can shape your
Common Lisp programs in a functional way, Common Lisp also support
Object Oriented Programming via CLOS, the Common Lisp Object
System. It is truly a multi-paradigm language.

This books teaches all the aspects of CLOS, and although written
several years ago, the knowledge still applies today. After learning
about Object Oriented programming using languages like C++ and Java,
learning about CLOS really opened my eyes to what this style of
programming should feel like. All the other languages now seem to
constrained compared to the power and flexibility of CLOS.

In CLOS, classes only have member variables, called slots, and support
multiple inheritance. They do not define methods. Instead, methods are
defined for Generic Functions which specialize on the classes. While
most OO languages use single dispatch, meaning methods specialize on
only one class, CLOS uses multiple dispatch. It also supports
specializing on distinct object instances. In addition to primary
methods, CLOS has before, after and around methods. You can even
dictate the order in which CLOS calls all of the applicable methods.

I truly enjoyed this book as it expanding my understanding of object
oriented techniques. I hope the mainstream language designers can take
the time to read this and incorporate more of CLOS into today's
popular, but limited, object oriented systems.