Ellie has always been in the shadow of her beautiful and talented sister. Lila was not just talented, she was considered a real mathematical genius and predicted a brilliant scientific future. But everything ended overnight. Lila was found murdered, and the crime was never solved. Twenty years have passed. Ellie has not come to terms with her sister's death; she still strives to understand why Lila died and who is responsible for her death. Ellie is now a professional traveler, purchasing coffee and visiting the most exotic places in the world. And one day, in a Nicaraguan village lost in the mountains, she unexpectedly meets the man who once knew Lila closest... And Ellie’s life will turn upside down for the second time. She will have to plunge into the past, into the secrets of her sister, which are closely intertwined with the secrets of unsolved mathematical problems. “You Don’t Know Him” is a novel in whose detective plot one can discern mathematical harmony, and at the same time it excites and excites, like the aroma of good coffee.