My two books of choice are:
The Cacti of the United States and Canada, 1983, Lyman Benson. It has over 1000 pages and is complete with distribution maps, b/w & color photographs, detailed line drawings, synonymies, uses, systematics, and most importantly, keys to ALL the cacti of North America, north of Mexico, including Hawai'i. Unfortunately, it are mo be expensive: I looked on Amazon.com just now and they have 2 copies available starting...STARTING at $247. That ain't cheap! Mine was given to me by a professor at Stephen F. Austin State University in Nacogdoces, Texas. God Bless Him!
The other one is regularly available in Texas:
The Cacti of Texas and Adjacent States, by Del Weniger. It is a revised edition of his book
The Cacti of the Southwest, another huge book like Benson's, from 1974... I used to have acopy of both, now I have neither. The current edition is self-described as edited for field guide format and covers Texas, Oklahoma, New Mexico, Arkansas, and Louisiana, and overlaps into those states. Lots of color photographs.
The only problems with these two valuable resources are that the systematics of the cacti are always changing, and some of the varieties in Benson are full species in Weniger, and vice-versa.
Anyway, I wish there were a photographic lexicon of the Cactales! But can you imagine how much that would cost?!
kari