It needs heat to flatten out. The easiest way it to put it out on a paved driveway in the sun with some weight on it, but you can also warm it up with a hair dryer and then put your books on it. It really needs the heat to soften up before it can be coaxed back into shape. There's lots of other tricks on different websites, but I found the hair dryer to be the easiest for me since I don't have a paved driveway or sidewalks anywhere near me!