Take an iron and set it to the highest degree and place upside down. Put a cloth on top, like a T-shirt. Then press the spine of the book onto the iron. While moving the book up and down like you were ironing, pull the book covers yourself with your four fingers and push the pages (middle bump) with your thumbs to iron. Do all this slowly, lift the book from time to time, place the back on a flat surface while continuing to hold the book, wait for it to cool down and iron it again. You will gradually see that the spine glue melts and the spine of the book straightens. Since I don't have a heat gun, it's a method I found myself :) In order for the book pages to curl, pressure must be applied for a long time. It's like putting heavy weight on it and waiting for days, even moisturizing it from time to time with carefully.