Systematische Anleitung zum Fantasieren auf dem Pianoforte, Op.200
Carl Czerny was Ludwig van Beethoven's student and in this treatise codifies the improvisation genres of his time, like the prelude, different types of fantasias, variations and more. It has MANY musical examples, which is why it's very accessible. It is very piano oriented, so for other instrumentalists it's inspiring, but there is a translation needed if you want it to work on another instrument.
It seems a translation exists:
An introduction to the treatise: