The works of Chozaburo Tanaka from Japan are also classic Citrus texts. They especially covered many obscure Citrus types unknown in the West.
Although he tended to name types as species, a trend that has been reversed to name all Citrus and relatives in the Citrus genus.
Genetic analysis has recently revealed complexity of Citrus evolution, including the male / female direction of crosses, multiple ancestors, back crosses etc.
This has left me with the impression that Tanaka was able to perceive some of this complexity now revealed, which is partly why he differentiated many types most would simply place together.