Here's the thirteenth book:
13. Shock For The Secret Seven - Derek Lucas Knight paperback illustrations 1973:
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Observations: Derek Lucas is first credited with illustrating the Secret Seven starting back in 1967.
It's now 1973. Is it me,or are the children starting to look a little older in some pictures?
This is the book where Jack has enough and clears off and quits the Secret Seven.
I notice Derek steers completely clear of external shed illustrations.
Perhaps he wanted to avoid any more confusion as to whether the shed had a fence either side, a tree in front,or behind the fence,or if it was connected to any other outbuildings!!