There is of course a sizeable literature in Japanese on Japanese castles, but the following list covers only works in English.
There is a handy little volume in the Hoikusha Color Book series entitled Japanese Castles. The author is Michio Fujioka. It was first published in 1968, and replaces the earlier Castles in Japan by N. Orui (Tourist Library Volume 9, 1935). The best monograph of the subject is Feudal Architecture of Japan by Kiyoshi Hirai (Heibonsha Survey of Japanese Art Vol. 13, 1973).
For the siege weapons used against Japanese castles see my books New Vanguard 43: Siege Weapons of the Far East (1) AD 612–1300 (Osprey, Oxford, 2001) and New Vanguard 44: Siege Weapons of the Far East (2) AD 960–1644 (Osprey, Oxford, 2002). The wider military background to Japanese castle warfare may be accessed through my previously published works such as Samurai Warfare (Cassell, London, 1996); The Samurai Sourcebook (Cassell, London, 1998); Campaign 69: Nagashino (Osprey, Oxford, 2000), which describes the famous siege of 1575 in great detail; Warrior 29: Ashigaru 1467–1649 (Osprey, Oxford, 2001); and Samurai Invasion: Japan’s Korean War 1592–1598 (Cassell, London, 2001), the first full account of this important campaign, including the only details in English of the wajô. A monthly update of my work may be found on the internet at www.stephenturnbull.com.
The best guidebook to Japan for anyone who wants to visit the castles mentioned here is Gateway to Japan by June Kinoshita and Nicholas Palevsky (Third Edition – Kodansha, 1998). For a spectacular scene involving the siege of a Japanese castle of the developed form, see the movie Ran (1985), directed by Akira Kurosawa. For a sengoku yamashiro, see Kurosawa’s older production Throne of Blood (1957).