Gwendy's Button Box by Stephen King and Richard Chizmar

Gwendy Peterson has set a goal for herself before starting middle school - she's going to run up what was known as Suicide Stairs every day to get in shape and hopefully lose the nickname of "Goodyear" given her by Frankie Stone. At the top, she pauses to catch her breath and realizes there's a man in a black suit and bowler hat sitting quietly on a bench, watching her carefully. Although she knows she shouldn't speak to strangers there is something non-threatening and charming about him. In spite of her initial fears, they begin to talk and he tells her she has been chosen to receive a very special gift - a beautiful mahogany button box. But this is no ordinary box, and with its rewards comes great responsibility. Over the next few years Gwendy realizes the power of the button box, at first inticing but eventually terrifying as her nightmares cross over into real life.