...allows federal agents to obtain a court order to access records of the books (and other materials) you use or borrow from this library for investigations to protect against terrorism. What kind of materials might they be looking for? ...Asking for a friend.
Generally they would want to know what a person has been borrowing and reading, which is why many libraries don't keep these records after you return the books. We can't surrender information we don't have, PATRIOT Act or not.