I have borrowed the following books from my Local Library, and they have a lot of interesting and useful information in them. You may want to see if your Library has them, or see if you can find them, or a current copy of them for sale.
Navajo Spoons: Indian Artistry And The Souvenir Trade, 1880s--1940s by Cindra Kline (copyright 2001 Museum of New Mexico Press)
Navajo Rugs: How to Find, Evaluate, Buy, and Care for them by Don Dedera (my library has the 3rd Ed)
One Hundred Years of Navajo Rugs by Marian E Rodee (my library has the 1st Ed copyright 1995)