I guess by number 16 the stories start to become a little dry and the setting a little too familiar. I have mentioned it before, and the reason is that others have also mentioned it: the Famous Five do not seem to grow up and that there seems to be an awful lot of holidays in England. In fact the Famous Five seem to spend more time on holidays than they do at school, and further, there seems to be many places around Kirin Cottage for them to explore (though I think there have been a couple where they have gone to Julian, Dick, and Anne's house, though they do not spend much time there).
However, enough of that because they are still an enjoyable read, and I doubt that Enid Blyton would have written 21 of them if people had become board after, say, number 10. Mind you, the Secret Seven books seem to take place over a much shorter time, and they are not necessarily on holidays, but they is probably because they all live in the same town and can easily meet up with each other. The Famous Five, on the other hand, live quite a distance apart, and also go to boarding schools, so they can really only visit each other on school holidays. Still, one would think that by this book they would have well and truly entered puberty.
This book is set during the Whitsunday holiday (apparently when they get a week off school, not that I know anything about that because here in Australia I never had a week off school during Witsunday, not that I actually know what Whitsunday is – we only ever had time off school during Easter, though when I was a kid we had two two week holidays between the terms and a two month holiday over Christmas and summer). Anyway, they decide to go and visit a friend of Julian and Dick's from school at a place called Billycock Hill were there is a butterfly farm and a secret airbase (not that they can go and run around the airbase because it is secret, and they do get told off when they go and have a swim in the pond).
As with some of the other books, a bulk of the story is about their little holiday at Billycock Hill and only the last third involves the adventure that they stumble upon. This one involves a couple of planes that are stolen and their friend's cousin is implicated in the theft because he, and his colleague, go missing at the same time the plane goes missing. As is typical, the Five stumble across the perpetrators of the crime, and inform the police of their discovery, and then Timmy locates (actually it is not Timmy but a piglet) the missing pilots and everything goes back to normal.
The pigling (as she calls it, though I always thought they were called piglets) was an interesting addition to this story. Basically the friend's little brother has a thing for baby animals (and they do live on a farm) so whenever a new animal is born the brother immediately forms an attachment to it: this time it is a pigling. The pigling unfortunately has a habit of running away, and the little brother tends to get into trouble when he runs off to find the pigling. However, this time the pigling saves the day, despite it being nowhere near as intelligent as Timmy.
As for Timmy, it is interesting that the TV series use this dog as Timmy:
when in the books, Timmy looks more like this:
I've also noted that Disney has decided to produce an animated series involving the children of the Famous Five but personally, I really don't think it would capture the essence of the original Blyton stories.