When people ask me what I'm writing right now, I always say I'm working on two new books, which is only half true. I edited one, set it aside, then edited another one. When that one is finished, I'll go back to the first one. So, I don't work on only one book until it's done. But it doesn't mean I'm jumping back and forth on a day to day basis. The key, I think, is being able to focus on just one, even if it's only for an hour or so. Tiny bits of progress, no matter how small, are still progress.