My favorite book about the "inner art" of Aikido is "Principles, Analysis, and Application of Effective Combat Throws", by Tim Cartmell. I happened on this book browsing in a bookstore one day. Mr. Cartmell, a student of Chinese arts, presents the inner dynamics of martial movement in a deep, clear manner. Physical and mental components of effective technique are included.
Highly recommended! Still in print. Unique Publications, ISBN 0865681767.