Re: Spruce Goose - nitty-gritty
I had to research the HK-1 for an article on the Martin Mars. A bit of Googling (other listing magazines are available...) will throw up what there is, which is a fair amount.
I can recommend "Howard Hughes and his Hercules" John J McDonald, 1981, Airlife, having a full tech spec, parts drawings load & perf diagrams, comprehensive history etc. Now OP, of course, but obtainable through the usual routes.
If you have a specific Q. try Evergreen Av Mus, that's where it is.
HTH