The Devil's Intern by Donna Hosie
When Mitchell Johnson was seventeen, he got hit by a bus and then he died. But, instead of that being the end of the story, this is the beginning. Since his death, Mitchell has been working in Hell's accounting office as an intern. It pays okay and is mostly filing.
He has three friends, of course where he is there's no time limit on who you can be friends with since everyone stays the age they are when they die, so his friends include a Viking and a peasant as well as a girl who died in the 1960s. They work during the day, hang out and eat fast food in the evenings.
Life, or afterlife would have continued for the group of friends except that the financial office where Mitchell works is in trouble. The Devil and his managers are ready to try drastic measures, including an artifact that can change. Mitchell and his friends have something they'd all like to change as well, how they died!
This is a very fun, very fast moving novel. If you enjoyed the Percy Jackson series or just get a kick out of time traveling, mythological fun, you are going to enjoy this great new book.