An X.Org book sprint is planned in the two days leading up to XDC2012 at the SUSE offices in Nürnberg, Germany. This book sprint isn't starting from ground zero but rather the developers that show up to participate will be working off of the documentation that was begun by Stéphane Marchesin
a while back (here's a PDF of where it's at today).
At the moment Stéphane's graphics driver book is 69 pages long. Current sections include the introduction, a look at the hardware, the picture, frame-buffer drivers, the Direct Rendering Manager, X.Org drivers, video decoding, OpenGL, Mesa, Gallium3D, GPU Computing, suspend and resume, technical specifications, beyond development, and conclusions. "The book will be targeted towards new contributors but will have enough details to be useful to all levels of graphics driver development."
According to the X.Org Book Sprint 2012 Wiki page the two days will mainly be spent reviewing the existing guide, filling in any poles, polishing the content, and then drinking beer. Originally there was an X.Org book sprint to happen last year at XDC2011 Chicago until the board's organizing fell through.
More information on the Nürnberg book sprint can be found in this mailing list post.