C# Design Patterns
Scott doesn't like "C# Design Patterns". I can't say that I do either. It was horribly written and very hard to follow. I will be looking into Slavomir's suggestion very shortly.
A book I would very heavily recommend is Wrox's VB.NET Debugging Handbooks. All the stuff they talk about applies to C# as well. I had never used automated debugging before, I've always been a build-test-rebuild kinda guy, debugging thru results, but it really taught me the power of the IDE's debugging capabilities, and it saves me tons of time on the development cycle.