Just a long and abiding interest in WWII US Naval Aviation . . . Books, magazines, reports, studies, manuals, academic papers, in print or digital, just about anything I can get my hands on that is relevant. And a special concentration on Midway related documents, including a few originals that NOBODY has.
The Library? For US naval aviation in WW2, I've probably well over 600 related official reports, histories, manuals, and documents, plus about another 500 official documents of just general naval interest or covering naval aviation of other periods. All plucked from the internet. In all, the physical library consists of some 500 plus books on military subjects, including well over 320 on naval aviation subjects, and another 75 or so covering the USN USMC, generally, USN USMC interwar, USN USMC WW2 and generally WW2 in the Pacific. The paper files have some 1450 document, reports, manuals, publications, studies, articles, and what have you, well over two thirds from official sources; and about 350 other periodicals mostly public, but some somewhat restricted in their availability general public due to membership requirements, such as, for examples "American Fighter Aces Association Bulletin" or "Foundation - Journal of the National Museum of Naval Aviation" or "The Hook - Journal of Carrier Aviation" or "Wings of Gold - Journal of the Association of Naval Aviation".
Like I said, anything I can get my hands on.