The unique concept. It combines time travel with a doomsday scenario. This makes it stand out from other science - fiction books.
Well, there are several factors. First, its plot is complex and engaging. It takes readers on a journey through different times and shows how they are all connected in the face of a potential doomsday. Second, the characters are very well - written. They are relatable and their development throughout the story is interesting. Also, the world - building in the book is excellent, whether it's the historical settings or the post - apocalyptic future, it all feels very real and draws the reader in, which contributes to its best - seller status.