This is the ultimate guide to C# 4 and the .NET 4 framework. Updated with more coverage of intermediate and advanced features, new examples, and detailed discussions of recent language and framework additions, this book covers everything you will need to know about C# and putting it to work. You will also find in-depth reviews of various topics including traditional Windows programming, working in Visual Studio 2010 with C#, base Class Libraries, and communication with Enterprise Services among others.
I own the 2005 and 2008 flavors of this book as well. It's the best overall C# reference I've found and this 2010 version is a welcome update. As with the 2008 book it could use a better guide to new features, but still very highly recommended.