Marcel Dupré needs no introduction - he wrote hours of amazing music for the organ. We often forget that he was also passionate about education, and wrote several works specifically for teaching purposes. Many of us started on the 79 Chorales, but he later wrote 24 Inventions, a further continuance of teaching work. They are of course more advanced than the Chorales, but still short enough that one can learn them without getting discouraged.
The first Invention in C reminds me a lot of Bach, and I enjoy it very much.