A few months after writing "PMD Applied" I felt that there were some things I wanted to add. Thus I wrote a supplemental chapter 11, "Better JUnit Tests With PMD". In this chapter I go through the PMD JUnit ruleset and show how you can use each rule to make your JUnit tests cleaner and more informative. Hopefully it will add more to the "PMD Applied" idea in that the chapter is devoted to, well, applying PMD to a particular domain - unit tests.
You can download the chapter for free (PDF format, 200 KB). Of course, it'd be great if you picked up a copy of "PMD Applied" as well!