Concurrent Programming in Java: Design Principles and Pattern (2nd Edition) by Doug Lea. A comprehensive work by a leading expert, who's also the architect of the Java platform's concurrency framework.
Java Concurrency in Practice by Brian Goetz, Tim Peierls, Joshua Bloch, Joseph Bowbeer, David Holmes, and Doug Lea. A practical guide designed to be accessible to the novice.
Effective Java Programming Language Guide by Joshua Bloch. Though this is a general programming guide, its chapter on threads contains essential "best practices" for concurrent programming.
Concurrency: State Models & Java Programs (2nd Edition), by Jeff Magee and Jeff Kramer. An introduction to concurrent programming through a combination of modeling and practical examples.
