Code Complete. This is a "coding best practices" reference. It's well written, though some of the techniques described are not always best applied to C# and .NET.
Pattern Hatching: Design Patterns Applied. This is a nice short book on how some of those great Gang of Four design patterns are actually applied in real life. A good supplement to the reference text.
Test Driven Development: By Example. Test driven development seems to be a methodology that has become increasingly popular in recent years. This gives you a good feel for what it is.
Windows Forms Programming in C#. This is a decent book if you want more detail on WinForms programming than C# and the .Net Platform, Second Edition provides.