Time to save Rebecca! Trust me, it's for the best, don't let her die. She's upstairs in the small library where you got the Dog Whistle, being attacked by a hunter. Hurry. After you save her, go back down to the study. You must turn on the lamp on the desk, and pick up the Metal Object next to you.
Leave the Metal Object in an item box if you can (head into the East Safe Room). You're going to fight the snake for the second time.