Riveting Writing

Last night my mother was telling me about a novel she's been trying to finish. She reads before she goes to sleep and this book, I guess, has her out like a light before even finishing one page. Several times she's woken up to find the light on and her book still in hand.
"It must not be very interesting," I said.
"Oh, it IS interesting!" she countered.
"Then why do you fall asleep?"
"I don't know!"
"What kind of book is it anyway?"
My mom starts laughing and answers, "Mystery."