I hope this is the correct place for this post. Zubalbooks.com in Ohio is moving and has a sale on until this evening with 10 to 50% off. If you click on Become a Member, you'll get a promo code for additional savings, which are shown for each item.
I searched for just "railroad" and got 481 really interesting hits: books on all RR areas of the US, limited Europe and Canada, short lines, interurbans, and lots of books on various logging roads and steel mills with RRs. There were limited WWI and WWII offerings, lots of legal and engineering publications, boxcars, GEEPS, train sheds, signals, electrifying roads, you name it. Oh, and a sermon about a fatal RR accident.
They also have books on model railroads and collections of magazines/RR Historical Society publications. The most expensive item was a travel guide showing RR lines from 1854. But most items were very reasonable.
Happy Hunting!
Tomlinson Run Railroad