I think there are three volumes of these books by David Myrick. I've wanted to get them for a long time and finally got around to buying a good used one, of Volume 2, "The Southern Railroads".
Have only had time to read a little of it so far, and skimmed through the rest, but WOW! What a great book! Lots of good historic photos of some very interesting and often unique railroads, and not just of the railroads and equipment but also the mines and towns. All of this will be excellent reference not only for those who might want to model one of these specific railroads, but also for anyone modeling mines and small towns.
The history of these railroads, mines and towns is also given and is quite fascinating, especially to anyone who has even a passing interest in such things.