I have read both JUnit in Action and Pragmatic Unit Testing (although I have the C#/NUnit version). These are both excellent guides to help you get started using JUnit effectively.
Once you have a general idea what you need to do, you will run into specific problems that you need to solve. For example, how do I test this legacy session bean? JUnit Recipes makes it /easy/ to find the answer to that question: just look in the index for "Test a legacy session bean" and there you find a 7-page article explaining your options in detail, including examples of how to do it.
JUnit Recipes is the kind of book you'll want to keep next to your keyboard for quick and easy reference.
Author of <a href="http://www.amazon.com/exec/obidos/ASIN/1932394230/ref=jranch-20" target="_blank" rel="nofollow">JUnit Recipes: Practical Methods for Programmer Testing</a>
"Pragmatic Unit Testing" was very basic in terms of actually getting into Junit. It was excellent on why you should use Junit. "Junit Recipes" sounds more like it covers specific things you are likely to encounter.