Re: If you could only have one Aikido book ?
This is a loaded topic...
I've read about a hundred different books (still have a few to go), and I still can't say what's the best. I guess it depends what you're trying to learn. "Dynamic Sphere" is considered by many as the best technical manual, and it certainly has the best illustrations EVER, but it is ponderous to beginners, and the numbering system for techniques is obscure. But it is a pretty good book if you consider that it was the "first and only shot" for the authors, who then promptly disappeared into relative obscurity 30 years ago...
"Aikido Shugyo" is excellent, as well as "Aikido & the Harmony of Nature". Some are more technical, some more spiritual. I suppose that in order to write a book, one has to theme it this way or that. "Enlightenment Through Aikido" is a new book that I found very interesting. Kanshu Sunamadori Shihan, like many of the older masters, waited until very late in life to put his thoughts in print (at least in English), as did Shoji Nishio Shihan, whose new book "Yurusu Budo" is also a great look at the internal side of Aikido.
On the technical side, both of Saito Shihan's sets are very good, as is "Aikido" by Kisshomaru Doshu, "Aikido in Training" (Crane), and Shioda Shihan's "Total Aikido". Someone also mentioned Shifflett's "Aikido: Exercises for Teaching and Training", which was much better than I thought it would be. Many of the available books also give insights into the differences between the various styles of Aikido.
Anything by John Stevens is thought-provoking, from his perspective as a "gaijin" with a lifetime invested in study.
For a LOT of history in a small space, you have to get Stanley Pranin's "Aiki News Encyclopedia of Aikido", as well as his "Aikido Masters".
For humor, there is "Angry White Pyjamas" and "Fudebakudo". And there is Terry Dobson's "It's a Lot Like Dancing", which is inspirational as well as funny. Not your typical Aikido book...
I guess that the point I've been circling around like a vulture is: Why try to settle on just ONE book, when there are so many, and something to be learned from each? Focus is a great thing, taken in moderation, but true knowledge comes from experiencing a diverse group of perspectives. And remember, YOUR path is yours alone.