"The Murder of Roger Ackroyd" by Agatha Christie. Majority of this book was the usual subtle evidences and eliminating of suspects for the most part, which were always a pleasure to read from Christie, but the real gem was the final solution. That was truly novel, and takes the mystery genre to a new level.
Actually I bought this book due to a brief mention of it in episode 11 of Hyouka, where Fukube Satoshi explained this meta-trope. If you watch Hyouka first, the killer is obvious from the very beginning.
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