Dealing with softcover textbook curling?
Don't try to repair the book. If you're an honest person, point out the damage when you return the book.

Libraries repair books. They have the tools and expertise to do it properly, without causing greater damage. They do it in a way which will preserve aesthetics and prevent future damage. And they treat it as a cost of doing business: they aren't going to yell at you, and except in extreme cases they're not going to bother charging you.

In the future, don't put things that shouldn't be bent (like softcover books) in an environment where they'll be bent (like the main compartment of a backpack).