Brazilian Jiu-Jitsu Reference

Selected Available Published English-language Sources:

Donn F. Draeger's books on Japanese bujutsu, budo, and Judo.
E.J. Harrison's The Fighting Spirit of Japan has sections on the early history of Kodokan Judo.
Katsuhiko Kashiwazaki's Shimewaza, Judo Masterclass Techniques from Ippon Books which contains some information on Ko-sen Judo, p. 72.
John Stevens' Three Budo Masters which has some information on Maeda, p. 37-38, and Kodokan Judo history.