Historian's Wisdom has an intervening "if" clause in its triggered ability. This means that you will draw a card only if the enchanted permanent is a creature with the greatest power both at the time the ability triggers and at the time that it resolves.

If the enchanted creature leaves the battlefield in response to the triggered ability but the Aura is still on the battlefield, the Aura will be put into its owner's graveyard as a state-based action. In that case, the last time it was on the battlefield, it wasn't enchanting a permanent, and its controller will not draw a card.