C# and the .Net Platform. This book is a great comprehensive introduction to C# and .NET. Its size is daunting, but it's written in a conversational, really easy-to-read style.
Framework Design Guidelines. Conventions, Idioms, and Patterns for Reusable .NET Libraries. This book has been nothing short of indispensable for what we’re doing. Written by two of the lead engineers on the .NET framework, it exudes authority and experience. They know what they're talking about. And they even solicit a number of other Microsofties for their opinions as well. The book even comes with a DVD that has various videos from internal MS seminars on framework development.
Design Patterns: Elements of Reusable Object-Oriented Software. This book is "old", but it still stands as one of the most relevant books around. This is the classic written by the "Gang of Four". The patterns described here have made their way into many parts of our design. If you don't get any other book, get this one.
Refactoring: Improving the Design of Existing Code. Many of the refactoring patterns in here are fairly intuitive and if you've coded for a while, you'll find yourself saying, "Hey, I've done that before!". But there's something about actually having a catalog of them that affirms your thinking. There are also some patterns that are not as obvious which are quite useful as well.