Aikido in Daily Life
...Otherwise, Tohei sensei has written a number of books on the subject. The one I like the most is not exactly focused on ki, but a delightful read anyway: "Aikido in Daily Life," from the 1960's.
I am not sure if there is an edition in print now, but surely it can be found used, and in some libraries.
has been revised under the title Ki in Daily Life
ISBN 4889960716 and is currently in print.
I would recommend two out-of-print books both by William Reed, one of K. Tohei's American students. Ki: A Practical Guide for Westerners
ISBN 087040640X and Ki: A Road Anyone Can Walk
ISBN 0870407996. Both are available online as used books.