Unit testing is obviously something all of us developers should all be doing...  and if you aren't, maybe a good book will help you start.

Roy Osherove is writing a book on Unit Testing (writing, as in he hasn't finished yet  )  But, you can get the first chapter for free, and the rest of the chapters as they are written via the MEAP (Manning Early Access Program).

Part I Getting started
 1. The basics of unit testing - FREE
 2. The first unit test - AVAILABLE
Part II Core techniques
 3. Using Stubs to break dependencies - AVAILABLE
 4. Interaction testing Using Mock Objects - AVAILABLE
 5. Mock Object frameworks - AVAILABLE