Website:
http://faculty.oxy.edu/rgrayson/

Professor of Music, Emeritus, Occidental College
Music Faculty, Crossroads School
Richard Grayson (born 1941) is an American composer and pianist. He is best known as an improvisor of classical music, most often with live-electronics. By the 1980s, he was regarded as one of the best non-jazz improvisers (Shulgold 1985). He was also organist at St. Martin of Tours Church, West Los Angeles until his retirement on May 31, 2009, after having served in that position for 28 years.

He offers an improvisation handbook for free download on his website here.

YouTube Channel:
https://www.youtube.com/user/improvelectronic/videos
While not improvisations at the organ, the collection of videos present wonderful demonstrations of different composer’s styles and how themes from one era can be treated in the style of a completely different composer.
Examples include:
“Singin’ in the Rain” in the style of Wagner
“Singin’ in the Rain” in the style of a Chopin Waltz
Moonlight Sonata in the Style of Khachaturian
Stravinsky’s “Rite of Spring” in the style of Mozart
Can Can in the style of Stravinsky
And this is only a sample!!!