Listed below are titles concerning the study and practice of improvisation that are available for free from assorted different on-line resources.
Books
C. Roy Carter
Theatre Organist’s Secrets
Arthur Clifton, 1784-1832
P.A. Corri’s original system of preluding:
Comprehending instructions on that branch of piano forte playing with upwards of two hundred progressive preludes, in every key & mode and in different styles, so calculated that variety may be formed at pleasure
Carl Czerny, 1791-1857
The art of preluding, as applied to the piano forte: consisting of 120 examples of modulations, cadences, & fantasias, in every style: op. 300
André Ernest Modeste Grétry
Méthode simple pour apprendre à préluder en peu de temps, avec toutes les ressources de l’harmonie
James Lyon, 1872-1949
Exercises in figured bass and melody harmonisation
Henry William Richards, 1865-1956
The organ accompaniment of the church services: a practical guide for the student
Hamilton Crawford Macdougall
First Lessons in Extemporizing on the Organ
Alfred Madeley Richardson, 1868-1949
Extempore playing: forty lessons in the art of keyboard composing
Frank Joseph Sawyer, 1857-1908
Extemporization
Hennie Schouten, 1900-1970
Improvisation on the Organ
George Tootell
How to play the cinema organ … a practical book by a practical player
George Elbridge Whiting, 1840-1923
Organ Accompaniment and Extempore Playing
T. Carl Whitmer, 1873-1959
The Art of Improvisation
Articles
Expertise in Musical Improvisation and Creativity: The Mediation of Idea Evaluation.
An article from the Public Library of Science by
Oded M. Kleinmintz, Pavel Goldstein, Naama Mayseless, Donna Abecasis, and Simone G. Shamay-Tsoory.
Dissertations
Michael Callahan
Techniques of keyboard improvisation in the German Baroque and their implications for today’s pedagogy