Expert .NET 2.0 IL Assembler rocks because the author writes well and includes every freaking detail you can imagine. It doesn't hurt that he wrote the IL assembler, disassembler and validator. Best of all you can buy it in a PDF instead of a dead tree.
Masochists interested in compiler development will also enjoy Compiling for the .NET Common Language Runtime by John Gough. I found this book immensely helpful during a virtual machine development project where I "stole" lots of ideas from the CLR design.
