Never read The First Five Pages, but saw it in a bookstore back when I was young(ish) and still dreamed of being a writer. (Stephen King's On Writing is all you need for that!) Since then, I've sort of adopted that as my litmus test for whether or not to purchase a book. If I can't make it through five pages, I am probably not going to enjoy the book very much.

The first line/page can stumble. It can be an over-thought gut punch designed to reel you into a book that never delivers beyond that point. Or it might be spent entirely on building sympathy for the guy who will be the first major character to lose his head. Five pages gives me time to feel the writer's rhythm, hear their voice, and get a glimpse of the world I am about to invest my time in. I know it's going to be a particularly good book when I find myself stopping more than five pages in - while standing in front of the rack!