Enid Blyton 
Five On Kirrin Island Again 
Famous Five #6

Kirrin Island belonged to George: it washer very own, and she felt aggrieved whenher father 'borrowed’ it for one of hisexperiments. And to make matters worse, he built a mysterious tower in the ruinsof the old castle.The Five couldn’t really understand Uncle Quentin’s wish to be left quite alone, and as it turned out, it was a good thingthat George took matters into her own handsone night-otherwise there would have beenno more adventures on Kirrin Island, and Timmy would never again have been able toprove how intelligent he was.This is the Five’s sixth adventure.