Programming .NET Security (available from O’Reilly, Amazon) is a great book which everyone should read.

It is possibly one of the best books introducing you to defense-in-depth in .NET runtime environments, assemblies, and classes. It goes into Role-Based Security, Code Access Security and how to practically implement it, as well as providing a very good overview of security in general, the different pitfalls you can fall into and the procedures and practices that underlie any successful implementation of security.

If you’re serious about security (especially in .NET), this book is a must-read.