Today, copyright can exist as chapters are being written, even if the book is not complete.
Registering drafts or single chapters is not only possible, but advisable, specially if they are to be shown publicly before completing the work. A judge may decide, in case of a conflict, whether there are rights over these paragraphs. In any case, it is always better to have evidence of such paragraphs than not. Such evidence, of course, supports and reinforces the registration of the finished work.