Currently, I am in the process of reading books about the Pennsylvania Railroad and its successors from my local library. A couple months ago, I found The Pennsylvania Railroad: A pictorial history by Edwin Alexander. The book is from 1947 and chronicles the first 100 years of the Pennsy. The name pictorial history is a bit of a misnomer by today’s standards because there just aren’t that many pictures in the 250-page volume.
More interesting than the pictures are Alexander’s accounts of the PRR’s history, which he lays out very thoroughly from its origins to its development and construction. Further, he describes passenger, freight service, and covers locomotive development extensively from steam all the way to electrics. He closes with a chapter of vivid description of incidents on the railroad including the Pittsburgh Riots of 1877, the Johnstown Flood and the Broad Street Station fire.
I found it a great introduction to the railroad and I was amazed at how many technical advancements were first introduced by the company. The book is out of print but maybe you can still find it at your local library or buy it used on Amazon.