The first sentence. The first chapter.
These are hooks that must latch into your readers eyeballs and force them to keep reading. We know that.
But guess what? You have to keep sprinkling little barbs through your prose to keep those pages turning. There's still the big question--Who did murder Mr. Body? Will the boy and girl get together?--but there has to be more than that.
Who had the revolver in the bottom of her handbag?
What did that mysterious letter say?
Why was Colonel Mustard in Miss Scarlet's room at midnight?
(Yes, I like Clue. It's the only game I always win! Mwahahaha!)
So have a great hook. But don't forget all those little barbs too. 'Cause if you do, your reader might forget to pick your book back up.