Like you, I have recently developed an interest in Kototama since it appears to be the foundation of O Sensei's road map to the development of Aikido - it is always good to place yourself in the mind of those you are trying to understand.
Anyway, one good source for more information is William Gleason's book, Spiritual Foundations of Aikido, which can be found on Amazon.com. This book delves very deep into the Kototama and is very esoteric in its explanation, but it is very interesting nonetheless. However, the best source I have found for detail information on Kototama is in the book 'Inochi' that was written by Masahilo Nakazono. This is an extremely fascinating book and goes into more detail than Gleason's book and it does include guidance for the actual practice of Kototama, it also includes sound charts. The book can be found at: