This is another very good series of books for girls. The books were put out in two sets, the first set from 1934 through 1968, consisting 30 titles. From 1972 through 1979 the series was revived with some of the original books reprinted and some new books. This group had only 17 titles. Counting only individual titles, there were 34 books written.
Louise and Jean are the two sisters who attend a private girl's school. There's a girl there, Lettie, who is their nemesis. They end up involved in various mysteries. The books are well-written and very interesting.