There's a new project that aims at producing Rails documentation as a series of guides called Rails Guides.
A wish list of topics to cover is published. People may pick one and write a guide, if that one is accepted he gets a prize. Details in the rules of the page linked above.
I think Rails will benefit a lot from this initiative, and I am very pleased to be a reviewer, together with Rails core member Pratik Naik and Hongli Lai, author of the outstanding Phusion Passenger and Ruby Enterprise Edition.
Writing this from Berlin, ready for the RailsConf Europe 2008 that starts tomorrow morning.