The Young Adventurers is a collection of books written by Enid Blyton, also known as The Riddle Series.
[1] The books included in the series were originally written as stand alone books or short stories, but they were revised and heavily updated by Enid Blyton's daughter, Gillian Baverstock, so that they featured consistent characters throughout and could be published as a series.
The Young Adventurers: The Riddle Series: The original books were: In this series, siblings Nick and Katie, along with various friends solve different mysteries.
In the book The Riddle of The Hollow Tree, it is told that Nick and Katie's parents have died, and they are living with their aunt and uncle.
At the end of the events in Hollow Tree, Nick and Katie have been adopted by their friend Laura's family, and Laura joins them in the remaining books in the series.