There are a lot of great books out there on the Rio Grande Southern. If you have the money, the 12 volume "RGS Story" is just packed with everything you would want to know about the RGS; and then some.
However, if you are only going to get just one book on the RGS, I would highly reconmend "The Silver San Juan" by Mallory Ferrell.
https://www.amazon.ca/Silver-San-Juan-Southern-Railroad/dp/0871080575
Its an older book but still a very interesting read with lots of photos that cover the whole RGS story from the begining in 1892 to the end in 1952. Mallory has done a lot of great books like the more recent "South Park Line" and "Rails Around Lake Tahoe", but I grew up reading the Silver San Juan and I still believe its one of Mr. Ferrell's best works.