Penny Nichols Series by Joan Clark
The four book Penny Nichols series was written by Joan Clark which is a pseudonym of the very prolific Mildred Wirt Benson between 1936 - 1939. It's a fast paced and fun little series and it's a shame that there were never any more written. It's unusual in the series book format in that Penny mostly works alone - she has a best friend Susan, but she is a peripheral character and doesn't really get involved in the mystery solving. There is also no 'boyfriend' character, and Penny seems to be a lot closer to her father than to the other sleuths in the genre. It's also a bit odd that Penny is only 15, but she has the freedom of the older sleuths like the Dana's or Nancy Drew - she even has a car and has apparently been driving since she was 12, I could understand this if she lived on a farm or in a rural area, but she lives in the city with her father! I can't imagine that even 80-odd years ago that 12 year old were allowed to drive and own a car:-) ...