All her life Ellie Enderlin had been known as Lila's sister. Until one day, without warning, the shape of their family changed forever. Twenty years ago, Lila, a top math student at Stanford, was murdered in a crime that was never solved. In the aftermath of her sister's death, Ellie entrusted her most intimate feelings to a man who turned the story into a bestselling true crime book--a book that both devastated her family and identified one of Lila's professors as the killer. Decades later, two Americans meet in a remote village in Nicaragua. Ellie is now a professional coffee buyer, an inveterate traveler and incapable of trust. Peter is a ruined academic. And their meeting is not by chance. As rain beats down on the steaming rooftops of the village, Peter leaves Ellie with a gift--the notebook that Lila carried everywhere, a piece of evidence not found with her body. Stunned, Ellie will return home to San Francisco to explore the mysteries of Lila's notebook, filled with mathematical equations, and begin a search that has been waiting for her all these years. It will lead her to a hundred-year-old mathematical puzzle, to a lover no one knew Lila had, to the motives and fate of the man who profited from their family's anguish, and to the deepest secrets even sisters keep from each other. As she connects with people whose lives unknowingly swirled around her own, Ellie will confront a series of startling revelations: from the eloquent truths of numbers to confessions of love, pain and loss.