Glean - Aggregates PMD + numerous other source code feedback tools
QALab - Aggregates PMD + Checkstyle + FindBugs and tracks problems over time
XRadar - Using PMD, CPD, and lots of other projects to give measurements on standard software metrics such as package metrics and dependencies, code size and complexity, code duplications, coding violations and code-style violations.
PMD Applied - the official manual; get an extra copy for the home! Or office! Whatever!
Checking Java Programs - by Ian Darwin; also discusses FindBugs.
Sustainable Software Development: An Agile Perspective - by Kevin Tate, discusses both PMD and CPD
High-Assurance Design: Architecting Secure and Reliable Enterprise Applications - by Cliff Berg, mentions PMD in a discussion of static analysis utilities
Eclipse Distilled - by David Carlson, discusses the PMD plugin
Maven: A Developer’s Notebook, co-authored by Vincent Massol, who’s done a bunch of work on PMD. Thanks Vincent!
Network Security Tools, chapter 6 discusses using PMD to catch web app security problems. Thanks to Joseph Hemler for the props!