Is your strap knotted? Is it the knot that is hurting your shoulder?
Can you move the knot off center to one side or another (front or back) so that the weight of your books doesn't fall directly from the knot but falls on a smooth stretch of handle?
Just a thought.
the knot is right at the top, so i guess that is probably the problem, but it's probably because I normally have way too many textbooks it my bag!
i was thinking of adapting the strap so it's maybe a little thicker. i don't know. i am going to see if moving the knot helps, but i think it's just how skinny the strap gets at the top.