You are reading the best of the genre, a classic of the highest level.And Then There Were None, is also a great book of Poirot. and The Murder of Roger Ackroyd

You should also try some "classic noir"
The Maltese Falcon
The Big Sleep
The Spy Who Came in from the Cold
These have been judged by critics to be the best mystery novels ever.
i think you can love also The Boys from Brazil. Is a great book different but great book.