This book is a slim one, just over 200 pages, and provides an easy introduction to writing unit tests. It explains the concept of unit testing, why they are beneficial, and carries on with how to write and manage tests.
The reader is gently introduced to the code, and shown how a unit test can be written without a test framework. NUnit is then introduced and is used to automate most of the gritty details. There is a basic overview of mock objects, and a mention of the NMock tool. The book briefly mentions test-driven development, code coverage, UI tests, and refactoring, but does not go into them in any depth. Which is fine, since this is intended as a focused introduction to unit testing.
For anyone new to unit testing in .NET, this book is worth a read. It does not go into much depth, but does the job as introductory material. If you are already comfortable with unit testing and NUnit, I can recommend The Art of Unit Testing: With Examples in .Net as a more advanced book.