It means you're as certain as you can possibly be on the given subject. Example: "What's the capital of Georgia?" "It's Atlanta." "Are you sure?" "As sure as I can possibly be." I'm an English tutor. Feel free to message me if you have any other questions.

It is a way of saying that the person is absolutely certain the spell book is there. "as sure as I could be" means that his certainty is comparable to the most certain he could ever be about something. Another example: "Are you sure you left your keys on the table?" "As sure as I'll ever be."