I have the Loetschberg-Simplon, Die Gotthardbahn (3 editions) and Bahnen der Voralpen books by the same authors. Yes, they are 40 years old or older, but they are filled with information about the routes and the history of the lines and have photos on every page.
They give you a basic understanding of the lines and possibly more important, you can learn translations by grasping the German, French or Italian text alongside the English. For example, if a photo description mentions kehrtunnel, you can figure out from the English text that it means spiral tunnel and the next time you see that word, you will understand what it means...
I found that the photos also provide a reasonable idea for some consisting for my model trains.
I would love it if somebody would travel the routes again and update the classic books with new photos...