While there are a number of references for Maven online, there is no single, well-written book that can serve as both a well-worn reference and an introduction.
Sonartype is going to change this, whith their online book "Maven: The Definitive Guide" It is a comprehensive reference and a narrative introduction to Maven. The Definitive Guide is written for both new and season Maven users.
The Definitive Guide is also an open source project and you are welcome to participate and contribute to the ongoing development of the book's content.
More information can be found on the maven defguide website.
Download your copy from here: