Mildred A. Wirt 
The Girl Scouts at Singing Sands [EPUB ebook] 
(Illustrated Edition)

The book revolves around Judy and her troop of Girl Scouts (Beaver Patrol). They are 8th grade girls (13? 14?) spending time at Pine Cone Camp with other scout troops. Judy's aunt telegrams her, informing her she wishes to spend a small vacation in the area and asks Judy to find her somewhere to stay. But it's summer, and everything nearby is booked. Well, everything except for a small, mysterious cottage.