Books on Patterns

Head First Design Patterns: A great first patterns book. Contains much more approachable discussions.

Patterns of Enterprise Application Architecture: An extension of the Design Patterns book.

Applying Domain-Driven Design and Patterns: With Examples in C# and .NET: A recent addition that synthesizes TDD, DDD, ORM, AOP, IoC into one text that is focused on .Net.

Design Patterns: Elements of Reusable Object-Oriented Software: The canonical book on software patterns.

Refactoring to Patterns: Surprise, its about using refactoring techniques to convert existing code to use Design Patterns.

Update:

Design Patterns C#: Thanks Tony