"C# 3.0 Language Specification" ebook
You can download a 519 pages long Word document providing the full C# 3.0 language specification from here. You can also download a 30 pages long Word document covering only the new features introduced in version 3.0 of the C# language (i.e. implicitly typed, extension methods, lambda expressions, object initializers, anonymous types, implicitly typed arrays, query expressions, expression trees, etc.) from here.
A nice set of hands-on-labs on the features of the C# languages is available here.
BTW it seems that extension methods (they make it possible to extend existing types and constructed types with additional methods) can already be used in .NET 2.0 projects. Read the final part of the MSDN Magazine article here for more details.
If you are looking for the specification of version 2.0 of the C# language, please read this other post.