Wow! I was happy to see thee previous parts of this interview being published, but now I am ecstatic. Thanks!
I think this is one of the best aikido lectures I have read-- the ones from the founder being usually my favorite. But, this one is much more explicit than those, regarding what a student should research to really start to understand. It is also consistent with some other stuff I've been researching (see below).

Since Tada Sensei brought up Buddhism, I would recommend this book to anyone interested in this.
I think it is really helpful especially from the point of view of the sentiment described in the lecture: that is, all religions being expressions of a single source, and Buddhism being central to the techniques of spiritual training in Japan. (Meaning this book may be directly applicable to an aikidoka's spiritual explorations.)

I am still trying to read it, but specifically, this book has two things of particular interest here: 1) the intro, describing the Buddha's own training and development of his understanding - very interesting when compared to this lecture and O-sensei's; 2) description of meditation techniques used and recommended by the Buddha - Dhyana is treated in great detail (spelled Jhana since the book uses Pali not Sanskrit) as being a tool for realizing one's unity with the universe. The point of entry for a beginner is also explicitly described as a breath-based meditation technique.