In this interview with the New York Public Library, Pico Iyer explains why it is best for writers to retain some sense of mystery:

Let this book hover somewhere between fiction and nonfiction. Let me give the reader no clue about how to categorize it before she begins or even after she’s finished. Let me put the reader on alert, on edge, not knowing what she’s going to get into.

If your reader has no idea what you are about to do, then everything can be a surprise.