While helpfull books on the suject are being recommended, I would like to offer up "Awareness" by the late Anthony De Mello. He was a Jesuit, but not bound by the thinking of 'his own team', he was interested in all paths to enlightenment. This little book was one of the most profound I have ever come across. Although there is no explicit connection to aikido, the connection is there in the focus on awareness itself.
For me the spiritual side of aikido does not exist in writings or words, it exists in the truth of the movements, deep connections can be made with another and oneself when performing the art. The more I practice the more I see/feel.