Yesterday I accidentally stumbled upon a goldmine: the Shared Source CLI 2.0 Internals book! This book covers the Shared Source CLI (if you don’t know what that is, check out the references at the end of this post) from an internals perspective. I find it extremely useful because a lot of what is written there is also relevant for the commercial version of the framework.
While the first edition was published by O’Reilly, it appears that the second edition [link here] never got published. So the authors, Joel Pobar and Ted Neward (both former Microsoft employees) have made the 2.0 book available for free on their websites.
Lastly, I’m not sure if you know, but we currently publish the ‘reference source’ for various versions of the framework. These cover most of the base class libraries, so it is a wealth of information. Check out the .NET Framework Libraries - Microsoft Reference Source Server!