Have been waiting for a more in-depth book on Kotodama for awhile, haven't been able to find much online.
Nakazono sensei, who was a very prominent aikido teacher, wrote several books about kototama/kotodama. They can be found here:
Of course, I also very much look forward to Gleason's coming book on the subject.
I wrote a chapter about kototama in my Aikido: The Peaceful Martial Art
, but I can't say that I "wrote the book" on the subject.
When I was a young aikido student, our teacher Toshikazu Ichimura held lectures in kototama, and we did some kototama practices in keiko, as well. I still do it, occasionally. Fragments, that is. Mostly on the vowels. The consonants are more cryptic, to say the least...