The Penny Nichols series consists of four titles: 1. Penny Nichols Finds a Clue (1936) 2. Penny Nichols and the Mystery of the Lost Key (1936) 3. Penny Nichols and the Black Imp (1936) 4. Penny Nichols and the Knob Hill Mystery (1939)Penny Nichols is the daughter of Christopher Nichols, an eminent detective. Penny's dad often disregards her ideas, but time and again she is proved right and he has to recognise that she is also a good detective. Penny is often accompanied in her adventures by her best friend, Susan Altman.These books have fabulous plots and are page turners for children of any age. Enjoy!