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 http://www.budovideos.com/shop/customer/product.php?productid=23778&cat=283&page=1
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.)http://www.budovideos.com/shop/customer/product.php?productid=22067&cat=283&page=1
Daito Ryu Aikijujutsu by Stanley Pranin (Pretty informative book, and probably the best one currently availible in English.)http://www.aikidojournal.com/catalog/productdetails.php?code=dtr
Books about koryu (old school) JMA in general:
Koryu Bujutsu: Classical Warrior Traditions of Japan (in 3 volumes) by Dianne Skosshttp://www.fightingarts.com/estore/catalog_books_classical.shtml
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?