Johann Sebastian Bach is often upheld as the master of German Baroque organ music, if not of all organ music ever written. For that reason, his music serves as models to study and represents a style for the improviser to master.
Book:
Pamela Ruiter-Feenstra
Bach and the Art of Improvisation
Volume One. Ann Arbor, MI: CHI Press, 2011.
Videos:
Latry – À la Bach – Notre Dame