Cool deal on restoring the D12. Those old allis's will always be the first Real orange tractor in my book.
I would say if you could find an old Allis Chalmers Shop manual or Parts manual will be your first bet to go with. The shop manual is far more in depth than an operators manual will provide and a parts manual usually has all the parts in exploded view and make things easier to cross reference if you need parts.
Check online for a PDF for sale or hope on ebay and do a search. Usually somebody will do it in reprint.
I have used I&T manuals and I'm not overly impressed with them. Usually they are printed for multiple models and may only have 1 chapter for your particular model of tractor.