What’s a book you read that was so darkly disturbing, haunting, that left you speechless?

Most of the books I read get boring. I’m particularly drawn to anything dark, disturbing, psychologically heavy, or just plain twisted. Books that explore the dark side of humans in a realistic yet haunting way

Also, I’d appreciate it if you could explain why each book is worth my time, as my "to-read" list is already extensive, and I often have trouble making decisions