Re: If you could buy just ONE book about Aikido techniques, what would it be?
I'm surprised that no one has mentioned the venerable "Aikido" by Kisshomaru Ueshiba. I haven't had a copy for some time, since I lent my copy to a student who then disappeared. The quality of the photographs in the paperback deteriorated from the original hardback edition, but it is all the same a very clear exposition of Nidai Doshu's conception of "standard" aikido. I also like the fact that the very young Chiba and Tamura senseis are taking ukemi!
Another classic is "Dynamic Aikido" by Gozo Shioda. Good photos and descriptions of Shioda's aikido, as well as some entertaining "practical self-defence" at the end - I particularly like the bit where the young lady in the tight skirt throws the big ruffian down the stairs...