For me, this is definitely the weakest book in the 'Adventure' series. There is nothing wrong with it as such, but there is also nothing original or unique to this adventure. The setting of the middle-eastern river is not as vividly or as excitingly realised as most of the others, and nothing happens in the story that we have not basically seen in the series already.In the previous book, we were forced to endure Bill walking headlong into a very obvious trap. Not only does the exact same thing happen in this one, but the children blithely do it too! I find this a very lazy and unsatisfying way to instigate the disequilibrium.Also, I find it very strange that Philip starts adopting the "Julian" role about halfway through the book, as he has pretty much always been in the "Dick" role throughout the series. Not only is this bad for Jack, who is forced to relinquish his leadership and become a slightly different (and weaker) character, but it's also bad for Philip, who suddenly starts talking like a pompous twit all the time!
