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
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