The Secret Series (Enid Blyton)

[1][2] Here are brief summaries of each story: Peggy and her younger twin siblings, Mike and Nora, are living with a harsh aunt and uncle after their parents are thought to have been killed in a plane crash.

In this first adventure, aided by Jack (a local orphan boy), they run away to live on a deserted island on a nearby lake.

They build a house made with the branches of a willow tree, and bring a cow and some chickens across, to provide milk and eggs.

In the course of the pursuit by the kidnappers, eager to get hold of him again, the five children visit the Secret Island from the previous book again, and hide there for some time.

The children, home from school and in the care of Dimmy, team up with Prince Paul, and go flying off in his royal plane to Africa (together with Paul's loyal and devoted courtiers, Ranni and Pilescu) to rescue the Arnolds – and get tangled up with a strange and secretive African tribe with flaming red hair and living within secret tunnels and caves inside a mountain.

Prince Paul, still attending the same boarding school as Jack and Mike, invites his four friends to his palace in Baronia for the holidays; but it is so hot there in the roasting summer that the entire royal household adjourns to their castle up in the mountains.

Paul invites his friends to Baronia once again, and they are camped near a secret airfield where new Baronian aircraft are being tested with the help of Captain Arnold; but there seem to be unknown enemies who are seeking to sabotage this work, and this seems to be connected with a remote, inaccessible valley that the children stumble into almost by accident.