There's a nice review of PMD Applied up over at JavaLobby. They give It a 9/10 and as always, the appendix explaining all of the rules is a big hit: "I recommend reading this without fail".

I'm glad folks are finding that appendix useful; there are a lot of notes about XPath there that can make it easier to write and optimize PMD rules. And with all the performance improvements that have been happening in PMD lately thanks to Xavier Le Vourch and Ryan Gustafson, XPath rules aren't the performance bums they once were. Good times.

For more on PMD, check the site or (of course) get the book!