I love books, I buy too many.  For me the best book, the one that I refer to several times a day, is C# in a Nutshell by Albahari & Albahari.  I discovered the free Linqpad program and decided that I wanted to reward the authors of this great piece of freeware so I bought the C#3.0 version of the book.  I quickly discovered that it is the right amount beyond beginner with the right amount of detail for me.  When the 4.0 version was announced, I pre-ordered it.  Now I have the 3.0 at home and the 4.0 at work and I get great use from both books.

Physically, they are a small form factor but they cover a lot of territory.  I refer to the linq and collections chapters a lot and always find the example code useful.  I can't say as much about the  reflection, stream IO or networking chapters because I haven't needed them as much, but when I have needed some help they always come to the rescue.

Basically, I can't recommend the O'Reilly 'C# in a Nutshell' books highly enough but I understand that this is a matter of personal taste.