I've been reading a lot of papers by Jim Mauseth, one of the world's leading authorities on cactus biology and anatomy and have found him to be very informative.
That guy seriously rocks.
His book A Cactus Odyssey: Journeys in the Wilds of Bolivia, Peru, and Argentina, James D. Mauseth 2002, is a great read reminiscent of the tales of early explorers of the South American wilds, such as can be found in various translated excerpts of individuals like Ritter and Knize in the cactus publication the Chilean.
I owe Mr. Mauseth a debt of gratitude for his contribution to the general knowledge of cacti.
Thank you Sir!