[MichaelBurge]

The professors don't actually make much money. The publisher takes a huge commission. I've heard 5-10% royalties in some cases. Usually if they use the book for their own course they'll waive even that to avoid having the appearance of a conflict of interest.

They could make a ton of money doing it this way: Physics textbooks can sell for hundreds. But many don't.

Using your own book for a course(separate from the money) has its benefits: The book has exactly the content that you want to teach off of, and you can also choose good problems.

When I took a Differential Geometry course in college, the professor had a translated version of some paper from a Russian mathematician back in the 1950s that he taught off of. It was cheaper to get copies from Kinko's, and it's not like the math changes. The old pioneers can sometimes give better intuition too.