It's funny, because many of my professors already do write what amounts to a small textbook, in the form of notes/slides. These are then freely available online

It's funny, because many of my professors already do write what amounts to a small textbook, in the form of notes/slides. These are then freely available online. By saving, editing, and expanding on these notes over the course of a few years, I'm sure most could have a fairly authoritative textbook for their course.