Hunt, Andrew and Thomas, David. Pragmatic Unit Testing. The Pragmatic Programmers, LLC. Raleigh, North Carolina, USA. ISBN: 0974514012
This book is a very good introduction to the world of unit testing. The first three chapters are basically an introduction to JUnit and can be skipped. The remainder of the book contains advice on how to write good tests. Unit testing is a very important tool, and I would hand this book to anyone who has never done it before. It won't take them long to read, and they will quickly be writing high-quality tests if they follow Hunt and Thomas's advice.