Figure I will leave this here.
Dr. Hall's book is printed, shipped, and available for sale.
It is a scholarly reference put together by one of the folks who spent a bunch of years in Japan training and
researching martial arts, particularly koryu. It is somewhat light on content regarding Aikido and other gendai arts, because there was a certain limit to the how big the book could be, and Dr. Hall wanted to make sure he provided entries for things that did not already have a lot of information available in English. Tons of excellent koryu-related content. The entries are a good mix of details about systems and organizations, terminology, concepts, and people.
If you have an interest in the overall history of Japanese martial arts you should really pick up a copy of this.