The Swiss Railways Society publishes two books that might fit the
requirements. "Swiss Travel Guides - 6, Graubünden" describes all the
lines of the RhB. It was published in 2013.
Published this year is another, larger, book "Swiss Independent Railways
- their electric locomotives past & present". There is a lengthy section
devoted to the RhB, including the former Bernina Bahn and Bellinzona -
Both are in English and are available from the SRS shop on their website.