Midnight Sun was illegally posted on the Internet out of Stephenie Meyer's reach. She was planing to have no one have the inside scoop on the new book until it was published and edited completely. She finds writing is not math, because 2 plus 2 always equals 4, no matter how you're feeling. But if she continued to write Midnight Sun now, James would probably just end up winning and killing Bella and the Cullens. No fan of Twilight would want that, so Midnight Sun is not making very much progress as we speak.
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Most people believe that Stephenie Meyer wrote Midnight Sun for her sister only. This is wrong, she started writing Forever Dawn, which was a sequel to Twilight for her sister, but it was quite mature, so she started over again with a whole new story, which is New Moon, now. She didn't to intended to have James lose the fight, and she wanted Bella to become a vampire in Twilight and James would die in the next book, but instead she made everything different, but we now have a better story and Bella eventually does become a vampire. Many of you already know that and have read that.