The Mystery of the Secret Room — Enid Blyton

In the cold and foggy Christmas holidays, it doesn't seem as though the Five Find-Outers and Dog will have anything exciting to investigate. Fatty's got some wonderful new disguises, but there doesn't seem to be any mystery for them at all this time. Then Pip stumbles on a single furnished room in an otherwise deserted house. They try to find out whom the house belongs to, but they're not making much progress - until Fatty decides to go to the house to see what he can uncover. Suddenly trapped by the men inside, he writes a note to his friends, apparently innocuous enough, but with another secret message hidden beneath it. Mr Goon has tried investigating the house too, but only ends up locked in the coal cellar and covered in dust. The rest of the Find-Outers call Inspector Jenks to help Fatty, and find the house has been a place for storing stolen goods and the men inside are wanted criminals. Another mystery safely solved!