Here are a few Daito Ryu books that give some history of this particular Ryu. There isn't too much in English that I know of. Remember that everyone has a bias.

Daito-Ryu Aikibudo: History & Technique Book by Antonino Certa

Discovering Aiki: My 20 Years with Yukiyoshi Sagawa Book by Tatsuo Kimura (this is almost completly Japanese, I haven't read it, only flipped through it.)

Daito Ryu Aikijujutsu by Stanley Pranin (Pretty informative book, and probably the best one currently availible in English.)

Books about koryu (old school) JMA in general:

Koryu Bujutsu: Classical Warrior Traditions of Japan (in 3 volumes) by Dianne Skoss

And check out anything my Donn Draeger, I know he talks about DRAJJ in Modern Budo and Bujutsu.

Anyone have anything else I've missed?

"Seek not to follow in the footsteps of the men of old; seek what they sought."