When trying to determine the value of anything, you need to be more specific. Is this a hardcover book or one of those small collections of images or postcards? Who was the photographer? The condition is always a factor.
Check out online auction sites as well as online booksellers to see if anyone is currently selling or has recently sold that book. You can check out AddAll Used and Out of Print, Bookfinder, or Books Antique Maps, Globes, Books, and Prints to search.
If you do find a copy of your book for sale, see if you can find a "sold" price. The asking price does not mean they will actually get that price.
Contact the Colorado Historical Society to see if they have a copy; it would nice to donate a copy if they don't have one.