The important book by ErichGamma, RichardHelm, RalphJohnson, and JohnVlissides about design patterns. Those interested in patterns or ObjectOrientedProgramming in general should check out this book and RefactoringImprovingTheDesignOfExistingCode.

http://www.amazon.com/exec/obidos/tg/detail/-/0201633612/qid=1034014007/sr=2-2/103-8276945-9875814?v=glance