Harmonizing: A Method to Encourage the Art of Improvising by Sietze deVries$35.98
A useful guide to harmonizing and improvising. It starts out with basic theory while gradually moving toward improvising in I, IV, and V in easier keys (C, D, F, and G). Later instruction includes harmonizing and improvisation in ii, iii, and vi as well as minor keys and even church modes! Published by Boeijenga Music Publications.