Nancy Drew: The Clue in the Diary by Carolyn Keene
In this mystery, Nancy and her two friends, Bess and George, drive by a house that explodes. Nancy pulls over and goes to the house to see if there is anyone inside, but the smoke is too heavy in the front of the house so she runs to the back and finds a man leaving the scene. This is where the mystery begins. Nancy and her friends meet a mother and daughter and somehow they are connected with the mystery. Nancy has to find out why the mother and daughter haven't heard from the dad and takes matters into her own hands. Along the way she has to find a missing man who is rich, but not liked, and why he has taken advantage of inventors and not paid them like he promised. This mystery will keep you interested and you won't want to quit reading.