The plague victim quest was designed so you had to decide who to save. The trick with stealing a cure potion back was unintended. After you tell one of them you gave away the only cures, they will not trade (or talk much, IIRC) with you. He is Sir Dante's son, but the father offers neither thanks or a reward if you save him.
I think you can use Telekinesis or a tossed teleporter stone to get that spell book.
