For two years, @WalterIsaacson lived in an Airstream next to Elon Musk while writing his biography. Isaacson didn't want a book built on a series of interviews. Instead, he got near-constant access to observe Musk firsthand. That's how he saw Musk's "demon mode" up close. Here's how he describes the mood swings: "He gets really dark and really difficult. He just goes silent." "There are people like Gwynne Shotwell and [Tesla design chief] Franz von Holzhausen who know how to push him out…