The devil's intern The devil's series, book 1.

It's been four years since seventeen-year-old Mitchell Johnson was hit by a bus and inexplicably ended up in the Underworld. Hell is miserable, but Mitchell knows things could be worse. After all, he has the coveted job of The Devil's intern-plus three close frirends who keep him from dwelling too much on his untimely demise. Still, he'd rather be living. So when Mitchell discovers that his boss is in possession of a legendary time-travel mechani...