I have been doing C# for five years and I thought I knew it all, and boy I was wrong. This is by far one of the most intense reading about C# (definitely for total beginners), it will take your knowledge of C# from a "user" level to someone who really knows what goes under the hood, for me personally, it opened new doors in terms of how I code.
.NET makes it far too easy to start developing applications without real in-depth knowledge, the entry barrier is much much lower than C++ (and dare I say Java), this comes with a drawback. The average programmer - like me - knows much less than the average programmer in those other languages. This book helps you take a step to separate yourself from the average knowledge. It goes into deep details of what goes in the CLR, the IL produced when you write code, performance issues, new constructs in C#4 etc... The result is a huge mindset shift, you don't simply write code that works but you actually think about how is it compiled and JITed, and you start thinking of using tools like the disassembler for the first time! Highly recommended.