I would recommend the blogs of Ellis Amdur on Aikido Journal... especially the "Aikido is Three Peaches" series. These are a good primer on what O-sensei's view of aikido was. (Not the end words, but a good beginning
Keep in mind that 'spirituality' in aikido is a very diverse thing. O-sensei was a member of a neo-shinto sect, and he happily mixes jargon from several different religions to make his point. As he doesn't always explain where he's getting this jargon, his writings often can look a tad... well, nuts to a western reader. But despite his religious beliefs, O-sensei never insisted it was a shinto art or something.
Other Japanese aikidoka, like Chiba Shihan, link aikido to Zen, and they do just fine. And others, like Shioda Koncho, were atheists and they did just fine too. My sensei, a 6th dan, is a deacon of his church (and his wife is the pastor).
Even more varied than what people think constitutes 'good aikido technique' and what people think constitutes 'proper aikido philosophy.'
oh, and dont worry about 'believing in ki.' what ki is and is not is an even more confusing argument.