This Book refers to Arithmetica written by Diophantus
Lisbeth suddenly understands the answer to Fermat's mathematical riddle - referring to Fermat's Last Theorem, which was written in the margin of his copy of the ancient text, Arithmetica.
Mikael Blomkvist, publisher of Millennium magazine, has made his living exposing the crooked and corrupt practices of established Swedish figures. So when a young journalist named Dag Svensson approaches him with a meticulously researched thesis about sex trafficking in Sweden and those in high office who abuse underage girls, Blomkvist immediately throws himself into the investigation.