What was I supposed to do?

As I took a book out of the shelves at the bookstore where I visit on a regular basis, I found someone slipped the wing of the book cover into the page where I think he or she stopped reading. I, for a split second, was bothered asking myself if it was right decision to take that book, because someone marked somewhere inside the book so that he could resume the reading next time he return to this bookstore. I, at first, thought finding another one could be a great solution, which ended in vain. What was I meant to do in that situation? Get it or not?