Yes, diaries would be best. Paper ephemera like payroll books, editor's notebooks, and the day-to-day correspondence of editors and publishers of GA comics would be quite a historical find. Not only would unknown artists be revealed, but the writers as well, many of whom may have been women.
In the GCD I often find credit alterations noted with " . . . based upon the editorial notebooks of Julius Swartz" or " . . . Giordano"
The Connecticut Historical Society recently obtained the payroll and cost logs of Eastern Color Printing....which includes material it published for comicbook companies from the 1930s and payment for specific titles
Now that's what I'm talking about. There's a book waiting to written by whomever wants to wade through all those documents for a year. If I lived close by, I'd do it.
Oh, and probably another year tracking down some of the people mentioned in the documents. Those that are still alive, or their families if they aren't, for clarification and elaboration.