Geography, early 20th century history (1900 - 1950s), transportation (trains, buses, streetcars) all that typical boring special interest crap.
What aspects of early C20th history? Social, military, entertainment or maybe technology? Anyway particular countries or societies?
Social, urban development, transportation development, architecture, pretty much anything from that time period. Mostly America, but any place really. If I hear about a place, I’ll probably start researching it’s history.
So your interest is pretty broad and open to expansion into other areas? That's a little unusual I guess, because most of what I've read online about special interest suggests that people tend to focus on specific areas of interest. I do have some interest in some specific aspects of the time period from the mid 1800s through to the late 1930s. I like the early forms of photograph, daguerreotypes, ambrotypes, tintypes and CDV cards and I collect some examples of those types of photographs. I also casually like art deco architecture and design. I'm interested in the Our Gang comedy shorts films of the 1920s, 1930s and to a lesser extent the ones from the early 1940s. I also like some of the pre-WWII male fashions. Oh and some specific members of European royalty, prior to WWI.
Opera (and before everyone says YECH!, this form of the musical stage, ESPECIALLY mid and late 19th and early 20th century Italian opera can be summed up in 5 words: Sex, Lies, and Video Tape! There's a reason daytime dramas on TV are called Soap Operas... My big influences as a vocalist are Jussi Bjoerling and Sir Jon Vickers), Oratorio, Art Song, Big band jazz (I started as a clarinetist, with Benny Goodman, and to a lesser extent, Artie Shaw, Peanuts Hucko, and Woody Herman as my inspiration. Too bad Carpal Tunnel Syndrome surgery has pretty much eliminated me ever getting the horn out of the case, as it hurts, in more ways than one, when I do.), reading, and dogs (especially Basset Hounds, Bloodhounds, Shetland Sheepdogs and Rough Collies (don't even turn on Lassie, lest you want to see the water works appear on my face.)