Improvisation still forms part of the foundation skills when studying the organ. So it is natural that there are modern day texts being authored on the subject. Organ also has a long history of improviser / composers that extend to present day. There are even entire competitions dedicated towards improvisation. It is serious stuff in the organ world. As for piano, especially classical piano studies, improvisation education tends to take on lesser importance.
Things might be changing, as learning materials for children are starting to include improvisation elements. That said, resources for pianists at more advanced stages of musical study are still far and few between. The organ books on improvisation are excellent entry points for pianists wishing a structured methodology for learning improvisation techniques.