George,
I was actually inspired to start doing this back in the early 90s after receiving C&W's "The Architects of Golf" as a Christmas present.
I was amazed at the number of courses in there that I'd played, even some quite obscure ones, but also then intrigued by courses I'd played that weren't listed and began my own search to find out shortly after.
Over the years resources have included contacting the courses directly, writing letters to folks who may have been around at the time, public libraries, historical societies, the USGA library, and in more recent years, online databases and most recently, subscriptions to online vintage newspapers and magazines.
It's sort of a cool "whodunnit", and of the 800+ courses I've played there are only a very few where I've been completely stymied. I've also researched a ton of courses I haven't played, primarily public, in PA, NY, NJ, DE, MD, VA, WV, CT.
Incidentally, some years back I did send my findings to Geoffrey Cornish to hopefully be incorporated in a revised edition of that book, and he wrote back a very nice letter. Unfortunately, I understand that the economics of another large coffee table book of that sort are not very good.