In the world of YA thrillers, Tiffany D. Jackson's Monday's Not Coming is one of the best. The story follows the disappearance of Monday Charles, who simply doesn't show up for school one day. Most unsettling of all is the fact that only her best friend Claudia seems to know — or care — that she's missing. This page-turner will keep you guessing (and your heart breaking) right until the end.