I encourage you to read Practical Common Lisp for an introduction to CLOS - the Common Lisp Object System. It's available for free online. (http://www.gigamonkeys.com/book/) If you are convinced that there may be something to object-oriented programming in Common Lisp that might be interesting and novel, I suggest two further advanced readings: Object Oriented Programming in Common Lisp and The Art of the Metaobject Protocol.

I use Clojure professionally, and CLOS is easily the thing I miss most.