Two of the Colorado RR Museum's "Colorado Rail Annual" books come to mind;
Narrow Gauge to Central & Silver Plume: Route of the famed Georgetown Loop. Colorado Rail Annual No. 10 by Cornelius W. Hauck
and
The South Park Line, A Concise History. Colorado Rail Annual No. 12 by Chappell, Richardson & Hauck
Neither one is listed on the Museum's website as currently available, though sometimes they have used copies. Copies also pop up on ebay and Amazon pretty regularly
Tome & Denise Klinger's recent books "C&S Platte Canon Memories and then some, "South Park's Gunnison Division Memories & then some" and "C&S High Line Memories and then some" are also good.
For what it is worth, I think that the CRRM books offer the most info for the least $$
Jason Midyette