The series first appeared as Riddle Series in 1997, published by Collins. The Young Adventurers version appeared much later in 2004.
In this series of story books for children by Enid Blyton, Nick and Katie, the twins, solve different mysteries around them. Somewhere in the series, we are told that their parents die from a car accident. But it is story of courage and strength, because the duo do not give up on their missions!
The other notable characters in this series are - Gareth, David, Sophie, Laura, Mike and Penny.