The Enid Blyton Dean & Sons Ltd ‘reward’ editions were the books du jour of my early 70s childhood. From memory, you could buy them at the supermarket, and I reckon every kid of my age had a few of these in their collection.
Unfortunately the illustrator of these memorable covers was uncredited… if someone knows who it was, I’d love to find out.
The Enid Blyton society website has lots of information about other incarnations of Enid Blyton’s books.