#1 The Secret of the Mansion by Julie Campbell, 1948.
Trixie Belden, a thirteen-year-old girl who lives with her family on a small farm near a town called Sleepyside, is sure that her summer vacation is going to be boring. Her older brothers have gone away to be camp counselors, and she’s stuck helping her mother with chores and watching her little brother.
However, Mr. Frayne, the strange old hermit who lives down the road from Trixie Belden and her family, is suddenly taken ill. Trixie’s father finds him passed out by the side of the road and takes him to the hospital. It doesn’t look like he’s going to survive, and rumor has it that there is a fortune hidden in his big, old, boarded-up mansion.
Then, another girl Trixie’s age moves into a mansion called the Manor House near Trixie’s farm. Her name is Honey Wheeler. Her family is rich, but she has lived a very sheltered life and has not had many friends. She has been unwell recently, and her family is hoping that living in the country will help her to get better. The two girls become friends, and Trixie convinces Honey to come with her to take a look at Mr. Frayne’s mysterious old mansion.
While looking around the property, they meet Jim Frayne, who is Mr. Frayne’s nephew. Jim has run away from his abusive stepfather and is now hiding out in Mr. Frayne’s old house. He had been hoping to find a home with his uncle, but he arrived after his uncle became ill. Jim is next in line to inherit Mr. Frayne’s property if he dies, but his stepfather Jonesy is looking for him and will take control of anything he inherits. Is there really a treasure hidden somewhere in the old house, and if so, can the girls help Jim find it before Jonesy finds him?
This is the first book in the Trixie Belden series.