THE REPTILE ROOM: by Lemony Sniket:

Violet, Klaus, and Sunny are the three Baudelaire children, who are sent to live with Montgomery Montgomery. Unlike Count Olaf (that wanted to steal their fortune) Montgomery was a good uncle. They were having fun, and a planned trip to Peru because Montgomery had to go there for work. Montgomery had a room full of reptiles and he studied them. Uncle Monty's new assistant, Stephano (who was really Olaf in disguise) came to live with them. The three tried countless times to tell Uncle Monty who Stephano really was. On the day of the trip to Peru, the children were looking for their uncle when they found him dead on his chair. Violet, Klaus, and Sunny knew immediately that Stephano murdered him. But the doctor said that he was killed by a snake. But those clever children found that the doctor was Olaf's helper and they proved that Olaf and the doctor were guilty.