I’ve been working with Maven a lot lately. One of my current projects involved migrating a complex Ant build to Maven using multiple modules, a ton of dependencies, and integration with a private remote repository hosted at Unfuddle (See: Subversion Help – Books, Plugins, and Hosting).
This book: Maven: The Definitive Guide has been indispensable throughout. In addition to the hardcopy, the folks at Sonatype are also maintaining an HTML and PDF version here.