Like Bryan, I think "The Spiritual Foundations of Aikido" by William Gleason is a really good book. However, I recommend "The Essence of Aikido" compiled by John Stevens instead. Mr. Gleason's spiritual, martial, and linguistic (knowledge of the Japanese language) skills are obviously great. It should be noted that his book is based on the totality of what he has learned and practiced. For example, the material on Kototama contains a lot of information taken from O'Sensei's book, "Takemusu Aiki," put out by the Byakko Shinkokai. However, the material is mixed with material Mr. Gleason learned from other Kotodama teachers, people who, as far as I know, have no connection to Morihei Ueshiba. As someone who has had some training in Kototama and has gone through the original writings of Morihei Ueshiba, this is for me, a big plus. Mr. Gleason is way ahead of me, so in reading his book, I learned a lot. The book is extremely difficult to understand, so it is definitely a book to read many times.
"The Essence of Aikido" is not any easier, but it is closer to a straight translation. John Stevens' other books, particularly "The Philosophy of Aikido," would probably be a better place to start.
I understand your post to actually be a call for discussion, not a request for advice, so I'd like to try to give my opinion.
Spiritual enlightenment seems to be such a subjective thing. From what I understand, it is often a thing to be felt in the presence of the person. Since we can't do that in this case, we have to look at what O'Sensei left. I think that it has been made very clear that a lot of what we know as "Aikido" has largely come through the influence of his son, Kisshomaru Ueshiba. To look at the Founder's direct teachings, the best source seems to be his lectures which were either written down or recorded.
My language skills are greatly lacking, but it seems to me that there is no overall complete teaching. When compared to other spiritual teachers, such as the Buddha, there is nothing clear and coherent. However, this could be construed as good when we see that the Founder had some incredible students who have become great teachers. Through them we can get some clues to the degree of the Founder's spiritual achievements. However, I think it is important to be aware of Kisshomaru Ueshiba's influence (as well as that of Kisaburo Osawa.)
I guess my main point is that in the Buddha's case, one can look at the basic teaching and be ignorant of Buddhist history and still come away with an appreciation of the Buddha's greatness. I don't think that is possible in Morihei Ueshiba's case.