Despite the title of this book, there is nothing mysterious about "ki." It is a concept popularly used in traditional psychology. Meaning variously "mind," "heart" and "spirit," it, like the equally vague "kokoro," postulates a quality that accounts for observed phenomena.
There are over 40 expressions using "ki," (a quick-tempered person is found to have a "mijikai ki") and Hiroshi Wagatsuma, the anthropologist, has listed all of them. He also notes (along with Takeo Doi, whose "Japanese Language as an Expression of Japanese Psychology" is required reading) that "ki" is always the subject of the statement itself. That is, the individual is not the subject -- "ki" is. It may belong to us, but we somehow belong more to it. It is as though it has a life of its own.
This is why "ki," like equivalent concepts in English ("soul," for example), can be used for many purposes, some having little to do with psychology. In the famous "manga" "Akira," for example, "ki" describes an energy supporting supernatural powers.