I don't seem to have had much luck just moseying around online. I'm looking for a book (or, I guess, books) that would tell me something about flare guns in use around the beginning of the last century.

Would anyone happen to have a suggestion off of the top of their head?

Here are two rather pricey ones.

FLARE GUNS & SIGNAL PISTOLS: THEIR USE, DESCRIPTION & ACCESSORIES


GERMAN FLARE PISTOLS AND SIGNAL AMMUNITION, STD