The 39 Clues (main series) is a series of eleven adventure books written by seven authors. In the series, the two main characters, Dan and Amy Cahill's grandmother, Grace Cahill dies. In her will, she says each person can get one million dollars or search for clues around the world and become super powerful. There are also online and card games. in book ten it states that all who pass through the gauntlet will get double the amount they turned down.