what does "as sure as I could be" mean?
How to justify the usage of "as sure as I could be" in below conversation? What's the expression in full?
A: Are you sure the spell book is there with him?
B: As sure as I could be

Question

Answer

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."

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Answer

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."