Some really good books will get false negatives by this rule, however.

For example: The Murder of Roger Ackroyd, by Agatha Christie.

By the bottom of page 50 I was not enjoying it at all, seemed like a trite mystery novel. I was ready to put it down but the friend who recommended it urged me to slog through. And boy was I glad I did. As is common with Agatha Christie, there is often a huge payoff at the very end (last chapter).