Hello all,
Because Progit (progit.org) is not freely licensed (the license forbids commercial use), and contributing to it is unnecessarily difficult, I have begun working on a textbook about Git on Wikibooks, and I've reserved space for a chapter on Gerrit: http://en.wikibooks.org/w/index.php?title=Git/Gerrit_Code_Review&action=...
If anyone would like to help with that chapter, or the larger Git textbook, I'd be happy to help you along as you get comfortable with Wikibooks. You can email me privately (I think you can see my email address), or you can leave me a message on the wiki: http://en.wikibooks.org/wiki/User_talk:Mike.lifeguard
Cheers, -Mike