I understand that "just write a book" might come off as glib, especially for profs who teach 3+ classes per semester. So I don't mean that everyone has to, or should, but

1. Writing 150 pages that are targeted for your class and your students is easier than writing "the bible" for your field.

2. The quality of the first draft might not be great, but if you are getting feedback and constantly improving, it's not long before you are better off than using one of the expensive tomes.

3. If you start with other free material, you can get off to a fast start (several people have now written books that started with my material, and then evolved beyond recognition).