Here's a start:

• http://www.artima.com has a plethora of good OOD information and enlightened interviews with smart, opinionated, people. This is one of the few newsletters I've ever subscribed to.
• http://c2.com/cgi/wiki?ObjectOrientedProgramming and if you want to get all object orienty up in there, try http://c2.com/cgi/like?ObjectOrientedProgramming as well...
• http://www.brent.worden.org/articles/index.html - where I learned about the Liskov Substitution Principle, the Open-Closed Principle, How To Love The Bomb, and various other For No Real Reason Capitalized Things
• Etc. I have to get back to work now x)

As a side note, I would emphasize that interfaces in Java also represent a sort of inherent 'contract', basically saying, "All objects of this type will have these methods, and those methods should do this". They impose a type constraint over the object, which in turn allows for polymorphism without inheritance, which in turn is a good thing. Inheritance breaks encapsulation by giving you intimate knowledge and access to your super classes' wibbly bits. The bottom line is you should usually give some thought to whether or not a class is going to be inherited from, and what will happen if it is inherited from. Otherwise, overriding certain behavior and not others can break code in unpredictable ways (this has bitten me on the ass enough times to make me worry about it).

Of course, there are many times when inheritance is useful, and I certainly do use it a lot. Superclasses are typically very bare abstract classes with only common behavior, however, and are not typically instantiated by themselves.

But that's just like, my opinion, man :).
Allen