We will open source in March, probably with a permissive license, but that's not yet definitively decided.
> Will it be able to reference page numbers from a different book dynamically? (so if book-a and book-b are in the same directory, one can reference another's sections)
This is not currently implemented, but also shouldn't be fundamentally impossible.
> Will there be support for intelligent floating images?
There will be floating containers (also with text flowing around). We will probably keep the amount of "intelligence" low for more predicatability. So you would specify top or bottom and it would be placed on the next page with free space.
> Will it accept 'every-page' commands, so I can use that #rect command to show a chapter's name on side-tabs?
Yes, every-page headers, footers, foregrounds, and backgrounds are already available and a way to query for the current chapter name is coming in a future update.
> Will error messages tell me hbox underfull badness 10000 at least 3000 times per compile?
No!