So the professor wrote his own book & then forced students to purchase it every year? Care to explain a little more?

I had the same thing happen at Georiga Tech years ago. My calc professor had the class buy a rough draft of the textbook he was writing. That wasn't as egregious as the situation described here, we could share books. The books were worthless at the end of the semester, though, since he'd have a new iteration out by then.

The book was as terrible as his teaching.

I had a calculus professor do this at the University of Maryland. Her book was awful. Her teaching method was standing in front of the class, and reading from her book. During one class, someone asked her to explain something a different way, and she explained that the way she was explaining it was the best way it could be explained, which is why she wrote it that way. The worst part is, the next year I had another math course where the professor literally did not speak English, and I couldn't understand anything he was trying to communicate, so she wasn't actually the worst professor I had there.

Very, very commonplace even 25 years ago when I was an undergrad.