​A Book You Must Burn to Learn the Ending

Imagine a novel where you must burn the book to reveal the ending. It’s a story about a couple struggling to have a baby. At the end they become pregnant. To reveal the baby’s gender you need to throw the entire book into a fire. If it burns pink, it’s a girl.

I’m not sure of the plot. I just love the idea of a book you can’t reread or resell.