Unified C# Language Specification
The Unified 3.0 C# Language Specification which combines the 1.1, 2.0 and 3.0 evolutions of the language into a single volume is available for download. This document covers every aspect of the C# language and apart from being free is also a better reference than any book you can buy.
Here you will find the definitive technical description of the C#
language in the words of the engineers who created it. It is the
single most authoritative reference for the C# language. [Link via Charlie Calvert]
This version of the specifications includes everything from the basic language constructs to more recent additions like partial methods and everything in between.
Direct download here or more information available on the C# Language page.