The Secret of Riverside Farm by Jean Hager

Twelve-year-old Alison Hill and her family leave their apartment in the city for a three-months' stay at Riverside Farm in rural Oklahoma.  Mr. Hill is a writer and plans to complete work on some manuscripts during the summer.  

Living on a farm is a new experience for Alison and her younger brother John.  They find the old farmhouse looking like a "milk carton"—tall, narrow, old, and dejected.  Alison feels a bit dejected herself and hopes the summer away from her friends will not be as drab as she expects.  

However, Alison's natural curiosity and love of mystery are aroused when she discovers a letter hidden in a window seat in her room.  The letter, yellowed with age, hints that something valuable is hidden on the farm.  

With the help of her brother and a neighbor boy, Alison begins a methodical investigation.  But clues turn up that are far more interesting than anticipated, leading Alison and her helpers into dangerous situations. 

I never cared about the story.  This book is missing that extra something that makes Jean Hager's The Whispering House special.  This book is just okay and nothing more.