Kyou Kara Ma no Tsuku Jiyuugyou!

Kyou Kara Ma no Tsuku Jiyuugyou is the novel the anime and manga are based on. It's about a boy named Yuuri, who gets himself flushed down a toilet into another world. There, he finds out he's the Maoh (demon king) and that the Mazoku (demons) expect him to lead them against the human race.
The novel is told from Yuuri's PoV, with plenty of side-commentary that is left out of the show.