Most likely everyone is familiar with it, but I would like to recommend Frederick Franck's book " an encounter with Oomoto, the great origin, a faith rooted in the ancient mysticism and the traditional arts of Japan (1975).
.......... the French philosopher Teilhard de Chardin and the words of Nao Deguchi and Onisaburo Deguchi. Reading the spiritual ideas of the founders of Oomoto kyo everyone can see where many of O Sensei spiritual thoughts must have had its origin.
It is a small book, but it is filled with nice details and interesting facts.
Frederick Franck (Dutch - American) is known for books like "The Zen of seeing", his art work, and the work he did with Albert Schweitzer. He is the designer/sculptor of the chapel of peace at his home in Warwick, N.Y.
All the best!
Many thanks Tom for this reference. I look forward to learning more about Oomoto Kyo through it. Teilhard de Chardin is one of my favorite 20th century theologians and my martial arts have never primarily been about fighting. Fighting is more the booby prize that results from the mindful discipline, creativity and bondedness between humans that martial training creates.