Quick disclosure - I'm a big fan of JUnit already. :) Anything that will convince java developers to use JUnit is a good thing. JUnit is as powerful as it is simple. To prove it, everything you need to know about JUnit is in this 80 page pocket guide. If you are a Java programmer, this pocket guide will easily convince you to add JUnit to your daily development processes. It covers the entire JUnit API, using JUnit with Ant, Extending JUnit, and plenty of suggestions on how to test various situations with JUnit.
I liked the descriptions on how to run JUnit standalone or within various IDEs (Eclipse, JBuilder and IntelliJ IDEA). I have been working with JUnit in Eclipse for quite some time and recently started a project using IntelliJ IDEA - so it helped me get JUnit going quickly. I also found the short history on JUnit interesting as well.
Whether you're new to Java development or just haven't used JUnit yet, this is the perfect pocket guide to help you get started.