e-dokusho.jp/e読書.jp (beta version) Allows you to search over 1 milion Japanese books both in print and digital. Provided by the National Institute of Informatics (NII).
Koji Ruien 古事類苑 An encyclopedic index of objects, events, and institutions appearing in classical literature. Based on the model of the Chinese encyclopedias. The work is divided into 30 broad categories. Each category has an introductory essay and relevant primary and early secondary documents.