This is a really cool book. Skim through it, and you'll see that all the hard stuff you want to do in .NET or C# are all in one book. You'll find some general computer science algorithms implemented in C#, and you'll find other things that are simply C# specific. I'd highly recommend any professional C# programmer adding this to their reference library.
The book contains a chapter for each of various C# objects. Beginning with numbers, the book continues through strings, classes, delegates, collections, I/O, threading, and XML to name a few. Some examples of "recipes" you'll find include how to improve string builder performance, issues with bit-shifting, adding notification callbacks, rolling back object changes, determining whether a process has stopped responding, and validating XML.
Like many of O'Reilly's other "cookbook" reference books, this book can be read from cover to cover, but it's really designed like a cookbook. The idea is to flip thorough each recipe in a specific section until you find one that fits what you're trying to do. While reading through this book, I came across five or six answers to various different problems I'm currently addressing in my own C# work. This is a definite must-have to your library.