or you mean that machine which has a laser to find out its price, if that I don't know it.
For security purpose they put some kind of tapes on the spine of each book. So if someone walks through the library gate without checking out the book the alarm goes off. Library staff demagnatise the book once booke is checked out to a patron so the alarm does not goes off. Again when someone returns the library book they make the sensor (tape on the book) active. They use a machine to make the turtle tape sensitize and desensitize.