There is a new Plone Book available: PloneLive 1.0 by Michel Pelletier and Munwar Shariff. What makes PloneLive really interesting is that it is a LiveBook that is constantly being updated. You can buy it print on demand for $44.95 from Amazon or a year's subscription for $29.95. The PloneLive Site also has a blog and other content. The authors and SourceBeat (the publisher) make a good point that traditional publishing mechanisms may not be able to keep pace with rapidly changing open source projects and dynamic books are the best way to document. Unlike a project wiki, the authors are compensated for their content. It will be interesting to see if this publishing model takes hold.
Unfortunately, the authors made a bad choice in selecting a sample chapter so it is difficult to tell if the book is any good. The sample chapter just superficially covers what a content management system is, what Python is, and how to install Plone.