The Silver San Juan-the best book on the RGS in one volume (Yes, I know that the RGS stories are great too), lots of rosters, photos and history. Mine, however is missing about 35 pages, not through being ripped, but a printing error. I guess that's why it was so cheap.
Rio Grande...To The Pacific-a great history about the politics, and engines, and lines. Rebel of the Rockies is even better for political, but Rio Grande...To The Pacific is probably the best D&RGW book to cover varied topics.