When I first started my university teaching, I thought teaching was easy. As a math teacher, I had a book selected by departmental experts and written by an expert. The department provided me with a syllabus for frequently taught courses. I was expected to selected the book and developed my own syllabus for courses in my specialty area.
The content I presented was mainly an explanation of what was in the book, but included an orientation of how I understood the material and how I thought it would best fit the needs and current understanding of the students. Students who did well on tests and homework got good grades. Those who did less well got lower grades. Some students failed mainly through lack of effort or through having an inadequate preparation for the new material being presented.
Over time, two things happened. First, I began teaching courses that (from my point of view) did not have satisfactory books. I took care of that by writing my own books. This habit started me on a path of writing more than 50 books. About 25 of these are available free on the Web at http://iae-pedia.org/David_Moursund_Books.