Well, this one was a little better than the previous Famous Five adventure that I read, however once again there is a little shift in the nature of the story, which is always a good thing because, since this is book number 18, Blyton no doubt has settled down into a pattern that seems to work (and I suspect the pattern will continue with the last three books in this series). Basically the pattern goes like this: the Famous Five are on holidays so they meet up and go to somewhere together (this time it is a large farm); the first half of the book has them running around having fun, and this part usually involves them meeting somebody that joins their adventure (in this book it is the twins); they then meet some rather unsavoury characters (the American and his rather annoying son); they discover that there is something nefarious going on; without the help of anybody they end up solving the mystery and saving the day; the Five, and their new friends, then finish the holidays having, well, more fun.
The thing that differs with this particular book is that there aren't actually any crooks, just some really annoying characters that are attempting to get a good deal so that they can profit from the transaction. Basically the 'bad guys', for want of a better word, (though in the end I would probably have to say that that description is actually quite apt for this particular people) are simply profiteers, and purchasing something at a bargain price because they know that they can make a huge profit from it.
I believe that these type of people may be known as carpet baggers, a term that came from a group that after the American Civil War went from the North into the South to assist in reconstruction, but in doing so would make a substantial profit off of the people in doing so. What the antagonists in this book are doing is that they have descended upon a farm that is struggling to get by on the rumour that there may be treasure buried somewhere on the farm, and paying what appears to be a substantial sum of money, but in reality is not all that much, to search for this treasure without actually telling the owners that the treasure is there. Thus, by offering them a small sum for the property they hope to make a large profit from it.
Technically there is nothing wrong with the practice, however seeing it from the Famous Five's point of view, one can see an essence of exploitation going on. The current owners are in financial difficulty and along comes an angel to bail them out, but the thing is that the angel is not all that angelic because they known that they can profit immensely from the transaction, it is just that they are not revealing the whole truth, and thus when the truth comes to light it is too late. However they are not actually doing anything wrong, which is why the police do not become involved, though these particular characters are painted badly so as to turn us, the reader, against them.
The one thing that I did not particularly like about the book though was the fact that they suggested that a castle that had been destroyed seven hundred years ago would still have traces left. I probably should not be so sceptical since one of my interests is archaeology. In fact suggesting that this is unrealistic when numerous sites that are much older have revealed even older objects inside then finding stuff well preserved in a cellar under a old Norman Castle is not so surprising.