I’ve thought about this quite a bit. It’s the one thing I can say all genres share. Every well written book in every genre is actually a mystery book.
What I mean is just about every author aims to leave the reader guessing by not being predictable. Mystery is mostly crime fiction at this point, but that’s an arbitrary classification. Any author who keeps their cards close to their chest throughout the story is actually writing a mystery.
Not every book is a mystery and not every author writes them, but I’d argue that every genre is full of mysteries and mystery writers. Even if these works are classified differently.
What do you think?