The Woman in Cabin 10 by Ruth Ware
The follow-up to Ware’s bestselling debut, In a Dark, Dark Wood, is just as much of a page-turner. Lo is on a luxury cruise around the Norwegian fjords, enjoying the chance to get away from her real life and a horrible break-up. But when she sees a body being thrown overboard, everything quickly goes awry, especially when it turns out that no passengers are missing. Tense and twisty, this is a brilliant update of the locked room mystery.