Lurker here for a few years but I have some thoughts on this topic ...
Aikido and the Dynamic Sphere, in the context of when it published (1970) and where (U.S.) is an interesting book in at least a couple ways:
1) It was one of the only Aikido books around in 1970 that was written by westerners who were also Aikidoka. As such, it offers a perspective on some things without being filtered through a translation.
2) It is one of only a handful of books that offers substantial amounts of prose on the subject, as opposed to only offering technique. (Other ones are around now, such as Ueshiba Kisshomaru's "Spirit of Aikido", Shioda's "Aikido Shugyo", Saotome's book (sorry, not in front of me so no title), and various works by John Stevens--but in 1970 this book would have been a stand out for that reason alone).
3) It is a remarkable work of illustration. In field guides and keys for the natural sciences, careful illustrations are always more useful for visual description than photographs. A well drawn illustration empahsizes the right things and can omit or elide irrelevant things. In a photo, there is often too much information, not all of which is relevant.
Having said all of that, it is not a book I personally rely on for much technically, since there are better books around now that reflect the increased diversification of styles, but its a classic work for the reasons above and probably deserves a place in any Aikidoka's library. Of course, not to suggest that collecting books has anything to do with Aikido.