I got these two books last weekend and both are excellent:

C# Data Security Handbook - a great read going through all of the support for data encryption, digital signatures and certificates in both .NET and the CAPICOM.  Full of excellent code-samples and useful explanations of the background theory.

Professional Design Patterns in VB.NET: Building Adaptable Applications - a good overview of the design patterns in .NET.  I would have preffered it in C# but it's easy enough to convert.  The start goes through about half of the GOF patterns and shows how to implement them.  The rest of the book deals with how patterns might help in different architectural tiers.  There's a good coverage on creating abstract classes for ADO.NET which allow you to use them without having to use a specific data provider.  I'm hoping to use this as an aid to a study group in future.