Slightly out of date: the classic Gang of Four book,
E Gamma, R Helm, R Johnson, J Vlissides,
Design Patterns, Elements of Reusable Object-Oriented Software Reading Massachusetts: Addison-Wesley (1995) ISBN 0-201-63361-2
Slightly newer, easy to read, uses a slightly old-fashioned
Java idion:
A Shalloway, J R Trott:
Design Patterns Explained: a New Perspective on Object-Oriented Design 2/e Pearson Educational (2005) ISBN 0-321-24714-0
[ August 25, 2008: Message edited by: Campbell Ritchie ]