Those who get past the improbable premise of Shepard's (the Pretty Little Liars books) series debut will find a fun and fast-moving mystery. Foster kid Emma never knew she had an identical twin until she sees an online video of a girl named Sutton being choked by a masked figure. She tries to contact her, but a planned reunion goes awry when Sutton never shows up (readers know this is because she has been killed; Sutton narrates the book as a ghost, unable to communicate with Emma, but eager to "solve my own murder"). Kicked out of her foster home and mistaken for Sutton by Sutton's friends and family, Emma starts living her privileged life; the stakes rise dramatically when she learns her other half is dead. Clique lit fans will recognize the familiar mix of label-dropping and mean girl behavior, but the dark mystery adds a compelling layer. Readers may have a hard time buying the ease with which Emma slips into Sutton's very different life, but they will race through the pages as Emma pieces together clues and will have plenty to ponder as they anticipate the next installment. Ages 14–up. (Dec.)