This book involves unlikeable one dimensional characters in a predictable plot. What Blyton fails to grasp is that to have any suspense in a thriller you have to have some doubt as to who the baddies might be. In the preceding (second) book the only significant character apart from the children and George's parents is the dog-hating tutor Mr Roland, who to no reader's surprise turns out to be the villain of the piece. In this book there are no other characters at all apart from the Stick family, who apart from having a silly name also don't like the children or their dog. One of them (horror of horrors) even has some pimples and isn't very good looking.
So it comes as a surprise to no one at all when the Stick family turn out to be the baddies. That's not to say the children are angels. Julian is as smug and arrogant as ever and is rude to the Stick son (for being smelly and not very good looking) even before the latter has been properly introduced as a character. Anne remains terminally wet while Dick has no discernible characteristics at all.
Add to this the fact that the dialogue and plots are so horribly dated, oddly much more so than a lot of older fiction. The lashings of ginger beer angle was well satirised by the comedy store but it is hard to believe that people actually behaved like this even in the 50s. Contemporaneous fiction like William and Jennings are also dated but at least the characters are credible and a little less one dimensional. When the baddies are locked in a bedroom at the end of the second book they don't break the door down, smash a window to get out or even shout and swear, they sit patiently in the room for 24 hours until the police come to arrest them.
The first book in this series wasn't bad, the second was worse and this one is just rubbish. It might have been Ok for kids 60-70 years ago but 8-10 year olds can read people like Anthony Horowitz these days and there is no comparison. Curiously the same author's 'Mystery of ..'/Five find Outers and Dog books aimed at a similar aged audience have worn much better. I'm not sure why this one is so bad but having read three of them to my kids at bedtime and found each of them significantly worse than the one before I'm not looking forward to the rest. Maybe this is just a particularly dud book...