Are you done yet?

The biggest problem with writing a book isn't knowing how much longer to keep going, it's knowing when to stop. Writing in a book long after it's finished, just because you're afraid it's not good, isn't a good idea. If you don't think it's ready yet, then either have another person read it, fresh eyes can do wonders for you. Or put it in a dark drawer for about a month, and then go back to it with new perspective. You're to close to the book now, take a step back and you'll be fine.