Five on a Secret Trail / Five Go to Billycock Hill (Famous Five #15-16)

Book 15, *Five on a Secret Trail*
The Famous Five are camping out near an old ruined cottage -- but is the cottage really abandoned? Anne hears strange noises at night, but the others don't believe her. Then they see the ghostly lights...

Book 16, *Five Go to Billycock Hill*
Hurrah! It's holiday time, and the Famous Five are spending it at Billycock Hill. Most exciting of all, they've made a new friend -- a real pilot! But now he has disappeared, with top secret equipment. The Five are puzzled, and concerned, too. Could their new friend by a spy -- a traitor? Whatever his motives are, the gang feel duty-bound to investigate.