I'm not making money from this that the author(s) could, so I'm not hurting the author(s) in any material way.

A: Most academics want their work used by students, and they are flattered when colleagues tell them so. However, they'd much rather hear, "My students love your book," or even, "My students had some very interesting reactions to your book," rather than, "One of my students loved your work so much she decided it was much better than what she could write, and so used it as her own."

More generally, authors are upset when they discover someone has taken their hard work, which often requires years of research and writing, and gained something by it without effort. It is, I suppose, a matter of stealing honor rather than money.