Five Go Off To Camp by Enid Blyton

A group of children and their faithful dog go off to camp in the countryside, where their holiday is interrupted by mysterious goings-on at nearby ruins and shoreline; using bravery, cleverness and a love of adventure they follow clues, outwit a gang of crooks, rescue someone in danger and help bring the villains to justice, all while enjoying the freedoms and friendships of life outdoors.