I caught the aikido history bug this week and did some research into two of Morihei Ueshiba's most influential teachers: Onisaburo Deguchi (Oomoto) and Takeda Soukaku (Daito Ryu Aikijujutsu).
If you have any interest in the origins of aikido and what O'Sensei was up to for much of his life, check out two great sources of info. In the first, a Jewish scholar studying in Japan wrote a condensed version/review of the book, The Great Onisaburo Deguchi.
Second, Stanley Pranin gives a 30 minute lecture on the history of Takeda Soukaku, the founder of Daito Ryu Aikijujutsu and teacher of O'Sensei. Pranin speculates that nearly 90% of modern aikido came from Soukaku's teaching. If you don't believe that, do a youtube search for daito ryu aikijujutsu and watch a few videos.