http://builtwithmortar.org/documentation/overview/concepts.html
http://builtwithmortar.org/documentation/overview/advanced_concepts.html
Neither of those are particularly done- I can certainly think of a few more things to add into it- but its a good start. I'm planning on Branching out from the overview section into specifics, probably starting with Modules.
Josh has also started work on the Theme documentation, as well as being having put the site theme together in the first place-
http://builtwithmortar.org/documentation/theme/themes.html
I think for the time being this is going to be my focus, although I'll keep poking at the 'general interest' style things since they are pretty important too. We need to kidnap some writers . . .
Robert