As an result of discussion on RedHanded about lack of good documentation for Rails, topfunky (host Rails podcasts – http://podcast.rubyonrails.org/, BTW I recommend to hear them!) have funded domain and VPS for Rails book (similar to Django book). It is http://www.railsbook.org/ (currently it is still domain parked on GoDaddy servers, so check it later).
Hurrah! Maybe it will become something more useful than api.rubyonrails.org. Kudos to topfunky!