I checked them out and many are available free for eBooks through libraries or assorted sources as their copyrights have expired. If you are looking for one of them, type in 'title author download'. They were quite readable online; but to download them to my Kindle, it was better to pay the $.99 that Amazon charged as straight scanning a book is often very unreadable. I bought George Custer's book as well as Elizabeth Custer's book, Boots and Saddles about her life as a military wife. I've been much enjoying hers as I am soon to start a book where her information will be quite useful even though it's Oregon and not the Dakotas and Montana.
L'Amour emphasized that he has used thousands of books and there are many more as good as these. When someone asks what is a good book researching the West, he asks-- what part, which states, what period because there are so many facets to Western history.
Commerce of the Prairies -- Josiah Greggs
On the Border with Crook – John Bourke
Life among the Apaches – John C. Cremony
Trailing Geronimo – Anton Mazzanovich
My Life on the Plains – George Custer
Warpath and Bivouac – John F. Finerty
Historical sketches of the cattle trade – Joseph McCoy
Fast and Fancy Revolver Shooting – Edward McGivern
Trail Drivers of Texas – compiled and edited by J. Marvin Hunter
Besides using journals and letters, he added that to truly write an historical western, you must walk the ground, climb the mountains, experience the land.Personally I think this is true of any book that is placed in a setting that isn't pure fantasy.