I had a professor in college that wrote his own textbook for our accounting course. It saved us, as students, a boatload of money. However having a textbook that your professor wrote is not ideal — it was much worse than any of the other textbooks.
We stilled learned the material, but did so by reading the book not learning from the professor — who just referred questions to specific pages in the book.
My experience is not going to be the case everywhere, but it is a very real danger to self-published textbooks.