QNX's docs[1] are some of the easier ones to read. The neutrino kernel briefly had it's source code available.
I think it was even MIT or BSD or similar?
They also had pretty liberal hobbyist/non-commercial license which let you learn the toolchain and develop with some of the middleware. That all went away when RIM (blackberry now) bought them.
Re: manuals - QNX Cookbook was also pretty good read. The documentation from that era were published as real books.