There are innumerable writing problems in an extended work....

There are innumerable writing problems in an extended work. One book took a little more than six years. You, the writer, change in six years. The life around you changes. Your family changes. They grow up. They move away. The world is changing. You're also learning more about the subject. By the time you're writing the last chapters of the book, you know much more than you did when you started at the beginning.