I wrote a Wikipedia article on William, others have added to it, you may want to start there.Next you could access a copy of the two books written about him by Herbert O. Brayer -- I was able to loan them from a library here in Canada.
William wrote a pamphlet about Colorado to entise investors and imigration to the area:"Colorado: Its Resources, Parks, and Prospects as a New Field for Emigration, with and Account of the Trenchara and Costilla Estates, in the San Luis Park. Published by Sampson Low, Son and Marston, 1869, London."
Searching the internet you'll also find that there are boxes of materials in the Archives of New Mexico -- possibly used by Mr. Brayer in researching his books.There are also several references to the collections of photographs of Native Americans that he commissioned.
I'm not a relative of his, but one of his relatives wrote to me and sent some materials -- I've tried to contact her recently but have not received a reply.