Below are a few recommended books to read :

Head First Design Patterns

URL: http://www.amazon.com/Head-First-Design-Patterns/dp/0596007124/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1215620872&sr=8-1 Description: A great book if you happen to be new to design patterns and OOP. The examples are simple real world type examples.

Design Patterns: Elements of Reusable Object-Oriented Software

URL: http://www.amazon.com/Design-Patterns-Object-Oriented-Addison-Wesley-Professional/dp/0201633612/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1215621046&sr=1-1
Description: A more advanced look on design patterns. I recommend reading Head First Design Patterns (above) prior to reading this. The authors of this book go by the name "Gang of four (GoF)" as these guys are some of the bread and butter of design patterns in OOP.