First lessons in extemporizing on the organ
This treatise stresses that hard work is most important when learning to improvise, always good!
It starts off with a sort of aptitude test, which is quite funny.
Generally it seems to be a very clear instruction book, but it does assume you already have some knowledge of how harmony works.
Starting from accompanying songs you know, it builds up from 4 bar forms to larger forms and eventually reaches a small intro to Fugue. It also has several musical examples in notation.