The Secret Seven Collection 4 - books 3 in 1

Book 10: Puzzle For The Secret Seven (first published in 1958)
In book ten, the gang witness the horror of a house going up in flames. Then there's the theft of a very valuable violin. Are the two incidents connected? The Seven mean to use all their detective know-how to find out!

Book 11: Secret Seven Fireworks (first published in 1959)
In book eleven, Jack's little sister, Susie, is so annoying! The Secret Seven are furious when they discover she's formed her own "detective gang"! Then some money is stolen, and Susie is a chief suspect. The Seven can't quite believe it - Susie wouldn't do anything that bad... would she?

Book 12: Good Old Secret Seven (first published in 1960)
In book twelve, there are strange happenings indeed at Torling Castle! Someone is hiding out in the ruined tower, making the resident jackdaws very unsettled. Who is it, and what do they want? The Secret Seven are on it.