Someone on the Samurai Archives online citadel needed some books or information on the Imagawa family. I have two in my personal library and used them in my bibliography in my book, Okehazama 1560.
Both are by Owada Tetsuo and are in Japanese.
Imagawa Yoshimoto no Subete. Tokyo: Shin Jinbutsu Ourai Sha, 1994.
Imagawa Yoshimoto. Kyoto: Mineruboa Shooboo, 2004.
Nobunaga no Tame!