If you are writing a paper on ki you'll need more than a good book! I would suggest looking at the chinese origins of the concept (i.e. chi), rather than just what aikidoka have picked up from Japan. I think it's a very adventurous project; maybe you should think of exploring just one small aspect of ki/chi - there are volumes and volumes on ki/chi in the area of health. You probably need an angle e.g. 'is there any proof for the existence of ki?'