An incredible resource for the DSP&P are Daniel W. Edwards' Documentary History of the South Park Line series. He includes only period news articles and company documents about the line. There is a lot of company information, but the news articles and stories from the era are a delightful read.
Here is
Volume 1 (of 7 I think)
You might find them cheaper elsewhere.
Another great resource for all the Colorado railroads are the back copies of the
Rocky Mountain Rail Report, though the focus is on preservation. There are still great stories in there of operating days occasionally. It goes all the way back to 1939. And it's free!
Kurt