Before getting this book, I wasn't really aware that there were narrow gauge street car or interurban railways in the United States. This book introduced me to the 42" gauge Los Angeles Railway and similarly gauged Denver & InterMountain.
I am not a trolley guy, exclusively narrow gauge up to a couple of years ago, when I took a trip to San Francisco and ending up riding various street cars of the Market Street Railway. And there, I learned about the Sacramento Northern, and the third rail power pick-up.
Just as an aside, The Interurban Era was part of a 3-volume set by Bill Middleton on electric railroading the U.S. The other two books were: The Time of the Trolley, which focused on the urban streetcar lines, and When the Steam Roads Electrified, covering the electrification of the Class 1 roads such as the Pennsy, New Haven, Milwaukee Road, and others.