Kyou Kara Maou! S

Yuri Shibuya just can't catch a break. First, he had his head dunked in a toilet. Then he was transported to another world - where he finds himself king of the Mazoku's, a new race that expects him to lead them to war against all of humanity. And to top it all off, he's engaged - to a man! 

(Source: VIZ Media)

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.