Japanese Castles gives us an overview of the history and architecture before giving us her first person impressions from her tours of some of the remaining castles. From explaining what a honmaru is to pointers about rest rooms and gift shops at the modern sites this is filled with useful information.
Sengoku Jidai Primer gives a great overview of the period for the non-historian. Special emphasis is given to people and events with an impact on the Inuyasha storyline.
These are indispensable to anyone writing anything set in feudal Japan.