Re: If you could buy just ONE book about Aikido techniques, what would it be?
My first choice for a book on aikido techniques would be The Way to Union with Ki by Koichi Tohei. However since it costs at least $250 USD and you need to have a Ki Society chief instructor verify your order from Japan, it's fairly hard to purchase.
A choice for a book would be This is Aikido by the same author, ISBN 0-87040-346-X. Since this is out-of-print and generally sells for $60 - $100+ USD, it is another unlikely candidate.
Another choice would be Aikido with Ki by Koretoshi Maruyama. This book is also out-of-print but has been selling cheaply on the net and in used bookstores, ISBN 0-87040-566-7.
Probably your best choice would be the paperback version of Aikido and the Dynamic Sphere by Westbrook and Ratti, ISBN-10: 0804832846 ISBN-13: 978-0804832847. It's a good all-around introduction to aikido techniques, while the above books are more specific. Used copies have also has been selling cheaply.