Most developers come across a reference to the GOF design pattern book at some point during their career.
Others refer to the Head first patterns book, or other similar books on the subject.
My advise: Instead of buying another paper weight and spending a lot of time extracting little information from a lot of pages (never could find the search option), go to the Dofactory design patterns web site. You can find information there easily and besides the theoretical data there are "real world" code samples in C#.
And if you are willing to spend some money, you can get further access to C# optimized code, as some patterns incredibly simple in C#:
- Implementing Singleton using lazy (or static) instantiation
- The observer pattern is simply an event in C#
- The iterator pattern is built-in into the language
So before spending your weekend with a technical book, take a look at the online alternative.