The Slipper Point Mystery and The Crimson Patch by Augusta Seaman

In The Slipper Point Mystery, Sally is bored and without friends until Doris arrives at Carter's Landing.  Sally is a bit of a tomboy, while Doris is very much a lady.  Sally is so taken with Doris that she begins to care more about her appearance.

Sally has a secret, which is a small room hidden in a nearby hillside.  Sally wants to share her secret with Doris, but she is afraid that Doris will abandon her for the other wealthy girls.  When Doris declines to leave Sally to go with the others, Sally realizes that Doris is a true friend and shares her secret, including a cryptogram that she found in the hidden structure.  The girls puzzle over the cryptogram and stumble upon a mystery that concerns an elderly woman who lives nearby.

From page 43:

Like many girls of her age, and like many older folks too, if the truth were known, Doris loved above all things, a mystery.

This is an excellent book.  The story is simple but entertaining.  The mystery is very interesting as it unfolds.

In The Crimson Patch, Patricia's father is home after being held a prisoner of war in Europe.  During the Captain's imprisonment, he learned about a German secret that is of great value to the United States.  Patricia and the Captain are staying at a hotel while the Captain works on a project involving the German secret.  The Captain has warned Patricia not to trust anyone and to tell no one about his story.

Patricia soon becomes friendly with Virginie, a girl who is staying across the hall with her aunt.  One day while Virginie visits, the Crimson Patch disappears from the Captain's room.  Two other people had also been in the room, so any of them could have taken it.  Patricia is heartsick about what happened and worries about whether Virginie could have been the culprit.  Chet, the bellboy, helps Patricia get to the bottom of the mystery.

This story is set up very much like the series books of the late 1920s and later, like Nancy Drew and the Hardy Boys.  The story is improbable, just like in the typical series book.  Patricia and Chet take matters into their own hands against the Captain's wishes.  They go off on what sounds like an ill-advised expedition to retrieve the Crimson Patch.  On pages 217 and 218, the Captain realizes that he should have told Patricia everything in the first place, especially since she did a better job of solving the mystery than he did.

This is a very good to excellent story.