I would be so grateful if you could find my favorite chapter book as a child! It’s about a friendship between two girls, perhaps a summertime friendship. They spend time in a tree house together. The paperback cover painting was in browns and greens of the girls in the tree house. Likely, I bought it through Scholastic Books when they had those mini-catalogs sent to schools. It came out in paper sometime in the mid 1960s.
Stump the Bookseller
New format, same book stumper solving.
Maybe The Secret Language by Ursula Nordsrom? http://www.goodreads.com/book/show/716097.The_Secret_Language
Mary’s secret by Emma L. Brock (1962) Mary and Mildred need a place to share Their Secret. At first the bough of the apple tree serves the purpose, but then father hits on a more elegant idea — a treehouse. And so with great fanfare the new domain comes into being. Even little sister gets into the act — as a sentry on a soapbox of her own. To celebrate the occasion, the girls hold a tree housewarming at which a nominal admission is charged for cookies and entertainment, both homemade. Suspense builds — for although readers will delight in the girls’ exciting summer in and around the treehouse, they are never let in on the secret. Of course a surprise ending is in store. The secret? — that there is no secret, just Mary and Mildred pretending.
Could it be Best Friends In Summer, by Mary Bard? Published in 1960, featured two girls named Susie and Coco.