Three Cheers, Secret Seven

The Secret Seven are Peter and his sister Janet, Jack, Colin, George, Pam and Barbara. They meet every holiday in the shed at the bottom of Peter and Janet's garden - and solve mysteries, and eat lots of delicious food. But no one can enter the shed without whispering the secret password! There are 15 Secret Seven novels, and a collection of short stories.
When Peter and Jack go looking for their lost model aeroplane in an old abandoned house, they find a lit fire in one of the rooms. So who is living there, and why
Looks like the Secret Seven have another interesting case to solve ...
Each book features rarely seen bonus Blyton: extra stories, a quiz, additional artwork and insights into Blyton's life and writing process.