If someone out there from Addison-Wesley is listening, hopefully they'll hear this plea to bring "Smalltalk-80: The Language and its Implementation" back into print. There is simply no better resource to completely cover this beautiful, unique system. Smalltalk is an extremely important language for both OOP and otherwise; there are volumes of concepts here that are remarkably relevant to this day, and yet we still have much to learn that our friends at Xerox PARC were able to accomplish 20+ years ago.
We like to think today that we know how to write books on programming, but I'll tell you, this is one of the most enjoyable books I've ever read on the subject. Surprisingly clear and thought-provoking, this book doesn't beat you over the head with the concept, but gives a basic idea and lets you use your own creativity in applying the material. Truly a book that appreciates the "programming is an art and a science" philosophy.
My copy of "Smalltalk-80: The Language" should arrive at my doorstep any day now :)