C# 5.0 in a Nutshell is a great reference book about C#. Furthermore, it is worth to read it from the beginning to the end, to understand how C# works and to get familiar with the language features (can be used in the future as a reference). This book describes many features, which I have never used (after over 4 years programming in C#) like some XML serializers, COM interoperability or advanced threading/tasking techniques. Now, I know about them and where to look if needed.
This book is rather for experienced C# programmers. I do not recommend it as “first C# book”. It has a lot of examples, but the structure is rather encyclopedia style, than discovering the language from the basics. For beginner I would recommend Sams Teach Yourself Visual C# 2010 in 24 Hours.
C# in Nutshell has 1064 pages! Although it takes a while to read, it is worth your time if you think seriously about becoming good C# programmer. There are very detailed overviews and analysis of the language, supported by examples. If you do not want to spend time reading entire book, I recommend to use it as a reference and start learning about concrete feature, which you need at the time, from specific chapter in this book.