Pragmatic Unit Testing in Java with JUnit

Pragmatic JUnitPragmatic Unit Testing in Java with JUnit by Andy Hunt and Dave Thomas (Pragmatic Programmer)

This book is a good introduction to the importance of unit testing. It gives a good overview of using junit as a testing framework. It also talks about areas of your code that should be tested and what conditions to test. If you’ve never done unit testing, this is a good introduction.