Whenever I deliver a training course on on low level debugging of .NET there are a number of books I take a along for people to browse during the coffee breaks (should they feel that way inclined ). These are books that I consider most useful for understanding how .NET works, rather than how to program applications in .NET.
Here is my current list:
Shared Source CLI Essentials
CLR via C#
Compiling for the .NET Common Language Runtime (CLR)
The Common Language Infrastructure Annotated Standard
Expert .NET 2.0 IL Assembler
Debugging .NET 2.0 Applications
Advanced .NET Debugging
HTH
Doug