Crouse27,
Generalizing first, I think there are two sources of information that have to be investigated. First, are there any standard "reference works" that would help? No matter what the subject, it is relatively rare that someone hasn't investigated facets of it before. In my case, some of the reference works and resources I've used so far:
1) As noted, "The Transports" by G.H. Davis
2) "United States Patriotic Covers of World War II" by Lawrence Sherman
3) Mellone's catalog of FDC's for the 1940's (essential, because everybody uses his catalog I.D.s, but I've found a couple of significant oversights or errors so far, so I will treat it with caution)
4) Postal Bulletins, which can be searched, here:
http://www.uspostalbulletins.com/pd...oup=48&id=485) The digital archive of back volumes of "First Days" which can be searched in their entirety from within Adobe Acrobat.
I'm sure there is a wealth of information that could be obtained via APS that I am completely unaware of.
The second source of information is whatever it is you are researching. This usually begins with what's in your own collection. Then you use Google to find other examples, at
ebay, in the online listings of auction houses, specialized web resources (e.g.
http://www.aerodacious.com/,
http://www.navalcovermuseum.org/, whatever the interest, there is probably a website dedicated to it), and so forth.
Stamp shows would be a kind of blend of these two. Exhibits, especially if they cover some aspect of what you are specifically researching, would be a source of knowledge and information. And then the dealers will have stock you can peruse to see what is "out there" that you are looking for. I attended my first show just last month, TEXPEX in Dallas. There were a couple of exhibits touching on airmail, and those were informative as to what and how philatelic research is presented, but they didn't touch on my current area of specific interest. Then I went around to the dealers. I came away with only one new C25a cover. The dealers that I talked to, and whose stock I went through, did not have anything at all like I was hoping for. But examples (of C25a and C26 FDC's on patriotic covers) show up on
ebay not infrequently, so every day or two I do a "newly listed" search of US covers with the keywords "patriotic" and "wwii." I've come up with a several nice covers this way. I won't give up on the stamp shows, but
ebay, and some online dealers (McCusker, Brookman, PostalHistoryStore, SwapMeetDave, Buckacover, to name a few) keep me busy searching.
I am a relatively new collector (started in 2003), but I've spent 40 years doing research in several different fields, and it is pretty much the same kind of endeavor for something like this.
Thanks for asking.
Basil