I like Outdoor Girl's response, offering to buy the textbooks from her, because you plan to highlight and write in them.
If she wants hers to remain pristine, and in her possession for the future, she can say no; and you go buy your own.
If she'd rather have the money than the books, she can say yes; and you buy them from her.
Or she might insist that you borrow, then return, them anyway, and treat them however you want. In which case you can refuse if you feel uncomfortable; or accept knowing that you told her upfront what would happen, and gave her an out.