Knowing exactly how the murders were committed and various other points (though some of them I only remembered about halfway through), I found a fair amount of the book rather frustrating. Perhaps it's just that I read an awful lot of these sorts of books and something that may well have first appeared in the 1920s is somewhat more commonplace now? I have to say if I were going to commit a premeditated murder, this is how I'd do it (and, if I ever do, this will be a terrible thing to have written, but since it's not a situation I anticipate finding myself in, I think I'm safe (and there again, it seems like it would be a good way to commit suicide, though without any hope of rescue, so one would need to be very sure of oneself)). 
 The point being, that I really enjoyed this book, but was a little frustrated with how slowly it developed and certain behaviour of the characters. Perhaps I'm just too suspicious. Still, I expect I'll read it again next year anyway.