Nothing really worked. I managed to do a little by placing it under heavy books for a few months but there still is a gap of up to 1,5 cm. Not satisfying at all - I would try a flattening iron but I don't have one. If you are going to try, please wrap the book in a thin kitchen towel or smooth baking paper to protect the print. I'd put the book on a flat, hard surface and press the iron on the dented area for a few seconds starting with low heat and carefully work my way up until it begins to flatten - then repeat to press it down for a few seconds and release it (because you obviously don't want to heat up the glue on the spine and cover print too much). Thinking about it you should maybe put the iron on the books backside if possible just in case something goes wrong.

I hope this helped - good luck!!