RandR Documentation
In addition to the RandR X protocol, an official configuration utility (named xrandr) is maintained in the freedesktop git repository.
RandR 1.2 overview from Debian
X Configuration from Ubuntu
Documentation from Intel on setting up dual head configuration with RandR 1.2
XrandR 1.2 HowTo from ThinkWiki
A Newbie's Guide from Phoronix
A short how-to on writing an X.Org driver, updated to include RandR 1.2.
For Historical (only?) value:
The usenix paper from 2001 describing RANDR 0.91 and it's motivations.