Four-fold Book Nomenclature:
"Textbook" - Resources that the student must purchase for this course.
"Required Reading" - Resources from which the student must read for this
course, but which are not required for purchase.
"Recommended Reading" - Resources from which the student would benefit,
but are not required for purchase/reading.
"Additional Materials" - Resources for the student who desires to
research the subject in great depth. Think bibliography.
I don't know, of course, the implications for you of implementing that
model, or the overhead involved.