I first was interested in Customizing the Microsoft .NET Framework Common Language Runtime by Steven Pratschner because I was looking for information on exactly how the CLR interacts with and could control execution of unmanaged code. It turns out, this is not a good source for that sort of information. Unmanaged code is mentioned only briefly, and it did not give the information I really wanted about that (any source of information out there for this? Leave a comment of click on Contact on the left).
However, the book does give a great deal of information about how to customize the CLR, as well as controlling exactly how it works. This is essential information if you plan to create an application that will host the CLR. Were that the only audience for the book, I expect that there would be a very small audience for this book. Thankfully, either intentionally or unintentionally, the book also covers how a number of Microsoft products I care for (ASP.NET and SQL Server 2005) interact with the CLR. For instance, people are often surprised that assemblies created for Stored Procedures and user defined functions in C# or VB.NET are actually stored in the database rather than in the file system. I imagine this will cause no end of confusion when SQL Server 2005 is shipped, however I completely understand why it was done. What I had not previously understood was exactly how it did it. Customizing the Microsoft .NET Framework Common Language Runtime explains this extremely well. It also covers some of the details of how IIS hosts the CLR for ASP.NET. While this information is not strictlty speaking required to work with ASP.NET, I always prefer to know exactly how a tool does something so that I can make the best possible use of the feature.
You may not need to know many of the details covered in this book, however, if you do need to, to create an extensible application that hosts the CLR, or if you use such an application and wish to understand what is going on under the covers, this book may be the only place to get the details you need or want. Well written and recommended.