Enid Blyton 
Well Done Secret Seven 
Secret Seven #3

Because the Society shed is too hot for their meetings, the Seven built a meeting place in a large oak tree in Windy woods. They tie large planks to the branches to sit on and bring cushions and find a nice cubby-hole in the tree to store their food. But they discover someone else is hiding in their tree – because of something he over-heard which had caused him to run away. He tells the Secret Seven what he remembers. A red pillow – MKX – a date, Thursday 25th, and Emma Lane. Will the Secret Seven be able to find out what all this means?