Most antiquarian fairs have websites listing the booth participants. Check that and see if any of the vendors specialize in that material. They might have links to the vendors websites which may give you a better idea of what they sell.

I haven’t been to the Washington Ant. Book Fair but every other book fair I’ve been to does have antiquarian book sellers specializing in 20th Century literature including 20th Cent. mystery literature. The 20th Century books are quickly becoming highly sought after especially when they’re 1st printings or have the first appearance of an iconic character like James Bond.

They’re great fun, and I wish I could go. Also talk to as many booksellers as you can, they’re a great source of information.

Guide to Collecting Rare Books : Collecting Autographed and Inscribed Books