John Mortensen's The Pianist's Guide to Historic Improvisation is a course-tested pedagogical method for learning to improvise, providing intermediate-level pianists with a path to mastery of the improvisational techniques of the 17th and 18th centuries.
Improvising Fugue: A Method for Keyboard Artists is a guide for those who aspire to the highest levels of fluency as inventors of spontaneous music at the piano, fortepiano, harpsichord, organ, or digital keyboard.