#3 The Gatehouse Mystery by Julie Campbell, 1951.

One day, Honey and Trixie decide to explore the old gatehouse on the property of Honey’s mansion. It hasn’t been used for years, and it is overgrown with vines. However, it looks as though someone has been there recently, and the girls find a diamond on the floor inside. There are signs of a scuffle, and the diamond looks like it was accidentally wedged into the floor. Have thieves been hiding out in that old cottage?

Honey thinks that they should turn the diamond over to the police, but Trixie wants a chance to investigate the crime first, just the two of them, and to prove to Jim and her brothers that she and Honey can be detectives as good as any man could. However, the boys become involved when someone attempts to enter Honey’s room at night to reclaim the diamond.

With their suspicions divided between the new gardener and the new chauffeur, the kids form a club they call the Bob-Whites of the Glen to investigate the matter and to trap the thieves.  This club continues for the rest of the series.

The book is currently available online through Internet Archive.