Without doubt, one of the finest reference sources on Aikido technique is the six-volume series titled Takemusu Aikido authored by Morihiro Saito, 9th dan, in the last decade of his life.
The Takemusu Aikido series contain hundreds of techniques that encapsulize the teachings of Aikido Founder Morihei Ueshiba in Iwama in the years following World War II. These wonderful reference manuals feature thousands of technical photos including historical shots of O-Sensei and Saito Sensei.