A little-known book that is interesting is:
'Desperate Men - Revelations from the Secret Pinkerton Files' - by James D. Horan.
First printed in 1949 - it covers a multitude of Pinkerton-related things, and is halfway well-illustrated with period photos.
It's been reprinted in softcover - but the original is a hardcover by Van Rees Press, printed by G.P. Putnam's Sons - NY.
An excerpt from the preface:
"We have received over the years many requests from authors and editors to examine these records. Although we aided them in specific details of their research as much as possible, we have refrained from allowing anyone outside of the agency to examine the complete files.
In the last few years, however, numerous motion pictures, books and magazine articles on the life of the James and the Youngers and other Western outlaws have appeared. Many of them glorified the bandits as misjudged Robin Hoods. To offset this erroneous conception, we decided to open our complete, century-old files dealing with Western outlaws for the first time to Mr. Horan. Although we have cooperated with him to the fullest extent, Mr. Horan's conclusions are his own."
Robert A. Pinkerton, II
Interesting stuff, and if your Impression is that of a Pinkerton, you should at least read the story...
Vaya,
Scouts Out!