Clever crime story with a quite typical stucture known from many other books and crime series (though it might have been a new concept by the time it was published). But I enjoy Poirot as a character and the solution was pretty nicely conceived which made it a good book to me (but there are definately better ones by Christie).

The concept was super intriguing but the middle of this dragged a lot and it seemed like it was going to be super disappointing for me. But the big reveal was super amazing and inventive. The real ending was a bit lackluster again and I would've hoped it went real tragic and creepy, but that's just my personal taste