The Penny Nichols series consists of four books: Penny Nichols Finds a Clue, Penny Nichols and the Mystery of the Lost Key, Penny Nichols and the Black Imp, and Penny Nichols and the Knob Hill Mystery. They are written under the name Joan Clark, another pen name of Mildred A. Wirt. They are a lot like the Penny Parker books, especially in the fact that both girls are named Penelope. As typical with her books, the heroine has no mother, a father who is in some kind of lawyer/detective/newspaper business, and a kindly housekeeper who is like part of the family. She has her own car with which she uses on her adventures and a best friend who is a little more on the plump side and not near as headstrong or fearless as her chum. All the books feature the same cover art. Descriptions of all four books can be found here. You can read them online at this website.
This post was originally posted on Phyllis' Book Reviews. It appeared on 4/10/11.