The Secret Series is a series of adventure novels written by Enid Blyton. There are five books, as follows:
A sixth book in the series, The Secret Valley, was written by Trevor J. Bolton and published by Award in 2009.
Here are brief summaries of each story:
Peggy Arnold, and 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 an orphan boy, Jack, they have befriended, they run away to an island on a nearby lake, and together they make a new home constructed with the branches of a willow tree, this much-loved "living house" being one of the highlights of the story.
However, living on one's own turns out to be far more difficult than they thought, and, along with a lot of fun and happiness, they also suffer a lot of hardship – especially when winter sets in. It does not help that the police are looking out for them, too, and Jack has to be very careful when he ventures ashore to sell produce at a market, and has one or two narrow escapes from being detained by them.
The novel was originally written as a serial and, in 1938, published as a full-length novel. It was one of Enid Blyton's earliest adventure novels, preceding all the The Famous Five, The Five Find-Outers, The Adventure Series, The Secret Seven, and school books which would appear in following years.
The children, reunited with their parents again (and Jack now adopted as part of the family), go on holiday at the rugged Cornish coast with a family friend, Miss Dimity ("Dimmy", who would appear in several further books in the series), and get drawn into a kidnapping case as they discover a boy being held prisoner in a castle tower nearby.
The boy turns out to be the Prince of Baronia, Paul, whom they rescue in due course, and protect as the kidnappers come after him in pursuit. Baronia is a small, remote kingdom of unspecified location (possibly eastern European), and Paul is heir to the throne. The Arnold children become firm friends with Paul during this adventure, and he appears in each of the remaining books in the series.
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. Paul's father, the King of Baronia, is very grateful to Paul's friends for rescuing him and then looking after him so well, and so impressed with Britain generally that he decides to send Paul to an English boarding school – the same one that Mike and Jack attend.