I have a book on my shelf somewhere about NT internal APIs; I forget the title or publisher as ive not looked at in nearly 20 years (thought it was interesting at the time, in practice I've hardly done more than glance at it). It's not officially sanctioned, and IIRC, there were notices that these were not considered public APIs and could change at any time without warning. It was from the Win2000 era, so who knows how relevant it is anymore.

Regarding the reverse engineering, ReactOS may be in a legal grey area. If they're disassembling, there might be some legal issues, bit I dont know, I'm not a lawyer and dont follow Reactor development closely (just checking progress from time to time when mentioned here or other places).