The following books, although not new, provide a wealth of information on Swiss narrow gauge lines. They are out of print, but Amazon offers used versions at reasonable prices:
Swiss Travel Wonderland, by Cecil J. Allen
Metre Gauge Railways in South and East Switzerland by John Marshall.
These British publications are very well done, and include many maps, gradient charts, and other data. While some locomotive information is out of date, I can't think of better guide to explore the marvelous Swiss railroad system.
If you go, purchase a SWISS PASS covering the days you will be riding trains.The pass is available here in the USA from RAIL EUROPE. It provides unlimited travel on most railroad, steamboat, and Post bus routes - along with urban transport (trams, busses, etc.). Mountain rack railroads are not generally included, but the pass does offer discounts ranging from 25 to 50 percent on the fares. One notable exception are the steam trains on the Furka Bergstrecke. However, that's a line you should include on your itinerary despite the hefty ticket prices. (There are a couple of other add-ons including the seat reservation fees on premier trains like the Glacier Express and the Bernina Express, but on the whole, the Swiss Pass is a great value.) And the best part is ease of use. You don't have to deal with language barriers at ticket windows - just hop on your train and show your pass to the conductor. The pass is available in 1st Class and 2nd Class versions.