That is how it worked in Pre-BB MapleStory: Wiz the Librarian takes one copy of the storybook when you finish the quest. You ended up with a spare because the book was handed out a second time while your first copy was not in your bag (in storage, for example), and the librarian still only takes one. The spare is safe to drop.

Chill, if you see this, I have suggested a thing to flag the safe to drop spares, if it gets implemented it will help you know whats good to drop or not :), you might wana keep them as a keep sake tho