Hallmarks of the Southwest/in Cooperation With the Indian Arts and Crafts Association by Barton Wright (1989, Hardcover)

This is an unmatched reference book for Southwestern Native American hallmarks and information on Navajo, Zuni, and Hopi silversmiths and their jewelry. I can't wait to search this book when I find a piece with an unfamiliar mark.
Short bios on each artist, with the year they first used their mark, help with identification. Often a hallmark will be a combination of initials indicating cooperation among several family members on individual pieces. Many of the old trading post and artist associations marks are also included.
There are more recent books, and they are very good for more contemporary artists, but for the collector of vintage or antique pieces you don't want to be without this one.