Secret Series by Enid Blyton

Four runaways, Mike, Peggy, Nora and Jack, find a secret hiding place—a deserted island on a lovely lake. They build a willow-tree house, make their beds of heather and bracken, and grow their own vegetables. And Jack even manages to bring his cow, Daisy, and some hens to the island for fresh milk and eggs every day! But one day invaders come to the secret island...

While on holiday by the sea, Mike, Peggy, Nora and their friend Jack go exploring in the caves on the beach. They soon discover a secret passage that leads to the Old House on the cliff, where they find a kidnapped young prince is being held captive! The friends make a bold plan to help the Prince escape - but will it succeed?

The parents of Mike, Peggy and Nora are held captive in the Secret Mountain in Africa. Together with their friends and Prince Paul, they run away to search for them. But can they find the mountain - and who are the strange and dangerous red-haired people living there?

What a wonderful, exciting adventure they have—Nora, Mike, Peggy and Jack—on their summer holidays. They go to Prince Paul's magnificent castle at Killimooin, high up in the mountains of Baronia. Little do they suspect that these mountains hold a dangerous band of robbers who are intent on capturing them and Prince Paul!

When Prince Paul's father, the King of Baronia, decides to rent a whole castle in England, Jack, Peggy, Mike and Nora are thrilled. Think of the fun you can have in a castle!But this is no ordinary castle. Strange lights burn in the locked tower room, weird noises are heard and books leap from their shelves!