Essential C# 4.0

This is an excellent book about C# the language. Please read that again, as I can see a lot of people mistake what this book is - it is NOT a book about .Net (although it definitely is related to that) or about any specific frameworks that _use_ C#, such as WPF or ASP.Net or anything like that. It's a book about the C# language itself. And assuming that's what you want, it's a really well done book. It also clearly points out what C# features are in/from the different versions of C# much, much better than any other book I've ever seen.
I'm not going to give an overview of the chapters, as you can already see what those are in the information section above. The coverage of every topic is very thorough, and the writing and sample code are both of high quality. If I had to complain about something here, it would be that some things which seem like "advanced" topics aren't marked as such all the time, while others are.
This book is a great reference, but you'll probably want it along with another book about something you want to _do_ with C#. You can use that other book (about WPF or ASP or whatever) to guide you along what you want to do and then use this one as a guide to C#. If you're new to C# or moving over from other languages (as I am), then you might want to read this first (yes, this is a long read, but it's worth it), then go to the other book, and only consult this one again when needed.
Oh, one last note. This book actually mentions MONO much more than other books like it, which is a nice touch.