Avi's The Man Who Was Poeis a tale of two mysteries. The first revolves around a mysterious man who comes to town. He is suffering from depression and even some delusion after the death of his wife. He says he is a famous detective, August Dupin, but when he begins to drink (which is often) he becomes someone else. In the end, the mystery is solved when he announces that he is Edgar Allen Poe.
The second mystery is the one Dupin/Poe is trying to solve--to find Edmund's family. Edmund meets the man after he has lost everything. His mother is gone, his aunt Pru is gone, and his sister is gone. The boy is distraught at his losses and stays with the drunkard longer than others might have. True to his self-proclaimed boasts, Dupin/Poe solves the mystery. Aunt Pru died trying to save her twin sister's life, and Edmund's sister was kidnapped and forced to help in a robbery. It is likely she is dead, but that remains a mystery. His mother is alive, and she is reunited with her son thanks to the detective/author.