A permanent love of mine although it has done some damage in the Opuntia world by describing O. occidentalis and O. demissa as hybrids with O. ficus-indica. There was apparently no proof for this other than phenotypic vibes.
The varieties of O. littoralis were fantastic and needed to be investigated more, but unfortunately the authoritative Jepson and Calflora have lumped them all back together.
A monumental work for it's time, I only wish that even more attention was given to David Griffiths account of the local opuntioideae.
This is the only field guide that I am aware of which focuses solely on the native cacti of California. This is an invaluable help when trying to identify various species in the desert regions of the state.