First, whatever became of of the FM which used to come with the software? Billy Gates and Co. halted it, so their Micro$oft Publishing Co. could reap profits from forced purchases by the noobies.
The very best documentation I've ever read --- and I suspect was written by engineers --- was the Perkins-Elmer OS 8/32 Processor Manual --- once this was read through and through (took me about 50 times) one completely grasped and understood the operating system at the hardware level/software level --- truly a beauteous proposition!
The next wonderful documentation was provided by Mark Minasi, on the private or commercial side, but I don't believe he is writing it anymore, but his books were top notch!
People to be avoided were technically ignorant types like Scott Mueller, who was simply a copyist, and would copy long lists of hardware parts (a great list compiler) but simply and obviously didn't understand hardware or computer sci.
But my experience --- and many others --- was that once outstanding inhouse documentation is written, the efftard bosses usually believe they can do without you!