I read the first chapter of this book while I was waiting at a train station. I read it like a couple of years ago but I think that at least 3 books of the series were published so I'm not sure when the first edition dates back. It should be a series of 3-5 books. I live in Italy so the cover probably is different from other countries as they usually do here.
The protagonist is a normal person of our world who responds to a job advertisement in the newspaper, only that the job in question is to become king of a fantasy kingdom.
After responding to the announcement he finds himself in this new world where he is told that he has 30 days to decide whether to be crowned and stay there forever or return to our world. The court magician mistakenly puts him under a sleeping spell, and in those 30 days, he passes like 27/28 asleep. Obviously, in the end, he decided to be crowned.