Peter Seibel wrote a Common Lisp tutorial that is the best introduction for Common Lisp that I’ve seen. I’ve poked around at a few different books, but that one was the one I actually learned Common Lisp from. It is clearly written giving you what you need to know without getting bogged down. I also find myself revisiting it as a reference from time to time even though I now use the Common Lisp Hyperspec much more. Common Lisp rocks.
Even If you aren’t looking to learn Common Lisp, I would recommend Object Reorientation: Generic Functions for a non-Java-like look at how object orientation can be done nicely. Multimethods rock.