Henry and the Clubhouse by Beverly Cleary

Henry and his friends are building a no-girls-allowed clubhouse. With a private space of their own, and a top-secret password required for entry, the boys are relieved to finally have a hang-out spot they don't have to share.

But Henry's about to find out that nothing—not even a sign—will keep gutsy Ramona out of their clubhouse…and her retaliation may just ruin Henry's newspaper career. For ages 7 to 11.