CLR via C# (Dev-Pro)
is a very good resource to master the intricacies of the common language runtime (CLR) and the .NET Framework 4.0. I think it is time to master C# and .Net Framework and this is one of the most recommended books on stackoverflow.
C# in Depth, Second Edition
is by far the most recommended book for learning deeply about C#. It assumes that you are already familiar with C# and its syntax and doing that allows it to get rid of boring introductory material.
When people ask for design patterns book for .net, they get Head First Design Patterns
. The book provides examples in java but they are relevant for .net developers as well. This book does not cover all the GoF Design Patterns but does a good job of explaining the patterns.
Professional ASP.NET Design Patterns
is more about layered architecture than design patterns. This book covers a lot of ground and is more suited for beginners to mid-level developers. David Hayden has a good review of the book here.
Refactoring: Improving the Design of Existing Code
is kind of natural progression from design patterns. Every application I have worked so far could have used a bit of refactoring and I intend to learn more about it from this book.
The Art of Unit Testing: With Examples in .Net
The only way I can make changes, refactor and still have confidence in my code is if I have a way of knowing that I haven't broken anything and Unit Tests are a way of doing just that - giving you instant feedback that all is well or NOT!