For those interested, this is a list of suggestions for basic reading material. The list isn't intended to be comprehensive, but to provide general background information. Unless otherwise noted, these books are available from Amazon. There is also a large collection of articles and interviews available online at the Aikido Journal website.
Many of these books (as well as many others) can be borrowed for free through the Hawaii Public Library system, which supports an online searchable catalog. You can submit online requests to have books delivered to the branch nearest you.
Best Aikido: The Fundamentals (Illustrated Japanese Classics)
Kisshomaru Ueshiba, Moriteru Ueshiba, John Stevens (Translator)
The standard text on basic technique from Aikikai hombu.
The Aikido Master Course: Best Aikido 2
The sequel to the above text, focusing on more advanced technique.
Takemusu Aikido Special Edition (Vol. 6)
A reprinting of Morihei Ueshiba's classic 1938 technical manual "Budo" with additional commentary by Morihiro Saito.
The Spirit of Aikido
Kisshomaru Ueshiba's explanation of his father's philosophy in more generally understandable language.
The Heart of Aikido: The Philosophy of Takemusu Aiki
Morihei Ueshiba, John Stevens (Translator)
Morihei Ueshiba gave a series of lectures to Masahisa Goi's Byakko Shinko Kai that are published in "Take Musu Aiki" (Japanese only) and remain the most extensive collection of the actual philosophy of the founder in print. This English version is an abridged version of the original Japanese classic.
The Art of Aikido: Principles and Essential Techniques
The English translation of "Aikido Shintai", containing articles on Aikido by Kisshomaru and Morihei Ueshiba as well as a section of technical photos.
Aikido and the Harmony of Nature
An explanation of the philosophy of Aikido by Mitsugi Saotome, a long time student of Morihei and Kisshomaru Ueshiba.
Enlightenment through Aikido
A collection of essays on the philosophy of Aikido by Manseikan Aikido founder Kanshu Sunadomari.
The Spiritual Foundations of Aikido
An examination of the spiritual underpinnings of Morihei Ueshiba's Aikido philosophy.
Aikido and Words of Power: The Sacred Sounds of Kototama
For the first time in English, the secrets of the sacred sounds that unlock the principles and spiritual strength of aikido.
Ellis Amdur's examination of Morihei Ueshiba, the man and the myth.
Hidden in Plain Sight: Tracing the Roots of Ueshiba Morihei's Power
*Available from Edgework
Ellis Amdur's examination of the sources of Morihei Ueshiba's martial power.
A Life in Aikido: The Biography of Founder Morihei Ueshiba
Kisshomaru Ueshiba, Moriteru Ueshiba
Kisshomaru Ueshiba's biography of his father, Morihei Ueshiba. The most complete biography available in English.
A series of interviews with prominent students of Morihei Ueshiba.
Aikido Journal - Daito-ryu Aikijujutsu: Conversations with Daito-ryu Masters
Stanley Pranin, editor
*Available from Aikido Journal
A series of interviews with prominent figures from the world of Daito-ryu, the primary technical source for Aikido.
An examination of the life of Onisaburo Deguchi, the spiritual instructor of Morihei Ueshiba.
Book of Five Rings: The Classic Guide to Strategy
The classic Victor Harris translation of Miyamoto Musashi's guide to military strategy.
Eiji Yoshikawa, Charles S. Terry (Translator)
Eiji Yoshikawa's novelization of the life of Miyamoto Musashi. Fictional, but a lot of fun.
The Unfettered Mind: Writings from a Zen Master to a Master Swordsman (The Way of the Warrior Series)
Takuan Soho, William Scott Wilson (Translator)
A series of letters from the famous Zen priest Takuan Soho to Yagyu Munenori on the nature of swordsmanship and budo.
The Art of War by Sun Tzu - Special Edition
Sun Tzu, Lionel Giles (Translator)
The classic Chinese guide to military strategy.
Ellis Amdur's survey of the martial traditions of Japan.