The classic "Nautilus" manual was really interesting on two sides; one was the quality of the writing, which was all down to Fred Butzen - he just had a real knack for clarity and working with him on documenting the work I did during the 4.0 process was really enjoyable. Basically the process was that developers would write up drafts of everything to give to him, and he'd then edit them for clarity before passing them back for an iteration to ensure that none of the accuracy was lost. He just had an excellent sense of when there was going to be a better explanation than the classic dry manpage style.
The other notable thing about that manual was just the economics of getting a perfect-bound book of that sheer size printed and included in the USD99 price tag.