The developer documentation now includes the current version of the OSMF Developer's Guide, in addition to the usual language reference.
It is still very much a work in progress, and is now publicly available here:
Also a PDF is available here:
Whichever version you read, we'd really appreciate your comments and suggestions, which you can enter at the bottom of any page in the HTML version. Please help us understand which sections are useful, and what else you would like to see covered that isn't there (at least in the outline) yet.