[1] http://www.amazon.com/Smalltalk-Best-Practice-Patterns-Kent/...
[2] http://martinfowler.com/books.html#refactoring
Lots of OO classes use it and Smalltalk is a great language to learn OO with (but even if you don't use it, the general information will be a big help).
[1] http://www.amazon.com/Smalltalk-Best-Practice-Patterns-Kent/...
[2] http://martinfowler.com/books.html#refactoring