"Aikido and the Dynamic Sphere" (Ratti)is perhaps my all-time favorite aikido book. One of the reasons I like it is because it does not use photographs of aikidoka performing techniques, but instead it uses sequences of drawings to portray the them.
The drawings are so wonderfully conceived that I think they do a better job explaining aikido movements than still photographs.
Another reason I like the book is that unlike many books written by shihan(which are also wonderful in their own way), "Aikido and the Dynamic Sphere" is not impregnated with the author's personal philosophy of aikido, but it has a rather simplistic approach on things such as movement and technique. This book is presented like a technical journal of what the authors saw and understood about aikido.
I consider Ratti's book and Ueshiba Kisshomaru's "Aikido" to be the most fundamentally helpful aikido books. Also Gozo Shioda's "Dynamic Aikido" is another great source. There are other good ones, bu I think these are the most useful as far as technique, movement, and basic principles.
Hope his helped.