Re: If you could buy just ONE book about Aikido techniques, what would it be?
I personally find little use for "The Dynamic Sphere", it didn't talk in a language that relates to my training at all. Generally speaking, no technical Aikido book I read did me any good. If you want to improve your Aikido technically, I suggest that a video is the better medium for that. However, both in a technical book and a video, you'd need some experience to understand what you're seeing, and take into account that what you learn in class and what you see in a video/book may conflict.
The first book I ever read about Aikido was "Aikido for life" by Gaku Homma, and I think while it's not a vey in depth book, and in some ways it might be a little juvenile, it did help me get into the right frame of mind early on, and it might do the same for you.