I recommend the Spirit of Aikido. Most things written about spirituality in Aikido and the philosophy of Aikido are very confusing to the western reader. It would take an education in eastern philosophy to understand them. I have found the Spirit of Aikido by Kisshomaru Ueshiba the easiest and most concise thing to read. I have read it once a year since I have been in this art and every year, it makes more and more sense. This year, since I have so many new students, I am planning a Sunday discussion class on it which we have done in the past. I am working on a study guide for the book that will be available for free in a few weeks to any one who wants one. It's what I will give my students for the Sunday class we are having. I am doing this especially for my students because I have become convinced that practicing the form of Aikido without understanding what the Founder intended its meaning to be will quickly cease being Aikido. It's the philosophy that gives meaning to the method of training.