Originally Posted by
Joseph Svinth
In English, articles by William Bodiford, John Rogers, and Harold Bolitho are worth finding. Budo Perspectives, by Alexander Bennett and Jigoro Kano and the Kodokan, edited by Alex Bennett, are also very useful.
In French, track down Henri Plee's material from the 1950s. It's dated, but not as much as you'd expect. Budokwai Quarterly Bulletin, also from the 1940s and 1950s, is even more valuable (though primarily for mid-century judo).
Ellis Amdur's three books provide a nice discussion of the development of the various aiki-budo.