http://www.livejournal.com/doc/server/
In particular, we're getting to some of the interesting stuff now:
http://www.livejournal.com/doc/server/lj.customize.index.html
It's still all very rough and we'd be incredibly thankful if somebody could volunteer to help with this all.
Help is needed with adding new sections, elaborating on existing sections, filling in missing areas (missing $LJ:: config vars, missing API docs, missing table docs, you name it...), and just general editing.
Any help at all would kick ass. DocBook is pretty easy to mimic and/or learn. I don't know it, but I just keep the reference open most the time to look stuff up. If you send us plain-text, that works too, and we'll format it for you.