Perhaps books are smaller in Hermione's world? Or are stored on data pads. Her purse seems large enough for one of those, certainly. Actual paper books are a bit of a rare thing in Spock's time so he does not jump to conclusions about that.

"An old, old friend. He was the captain of the ship we were both stationed on for many years. We were separated due to a situation beyond our control, but I've recently discovered him living here in the Nexus."