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.
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.