Agatha Christie's best-selling novel is 'And Then There Were None.' This masterpiece has sold over 100 million copies worldwide, making it one of the best-selling books ever. The plot is brilliantly crafted, with ten strangers invited to a remote island, only to be killed off one by one. The suspense is relentless, and the twist at the end is unforgettable. Christie's ability to create tension and mystery is unparalleled. The characters are vividly drawn, each with secrets that unravel as the story progresses. It's a book that keeps you guessing until the very last page. The novel's popularity is a testament to Christie's genius in the mystery genre.