Way back in July, I mentioned a book titled They Captured the Moments: Dakotas Photographers 1853-1920 (see post). I stopped by the local library to see if they had a copy. I found the book, which is good, because I wasn't really interested in paying $45 for the book without seeing what it had to offer.
It has information on several photographers (I think they said 3,000+), for the Dakota Territory area. As long as I had the book in hand, I responded to a handful of queries on the Ancestry.com forums regarding photographers in the area and timeframes they were in business.
The book also notes what types of pictures would likely be found in the Dakota Territory area and during which time periods those types would have been prevalent in the area. For example Carte-de-Viste style photographs, which were usually produced on 2-1/2x4 inch cards, gained popularity in France in 1854. However in Dakota Territory they were not commonly used until the late 1860s and then were only used in the area until around 1890.
All in all, I think They Captured the Moment: Dakotas Photographers 1853-1920 would be a good reference for someone that had a large collection of photos they were trying to identify.