Document dis-assembly can be nothing more than highlighting a piece of text that you want to save as a separate clause, pasting it into a new document, and then saving that new document into the appropriate folder. The next time you display a book using the Document Assembly button, the new document will instantly and automatically display alongside the other documents in the folder. (If a DropDown List is using that folder as the source of its content, you may have to 'Refresh' the DropDown List for the new document to show. But this is a one-time only refresh.)
The dis-assembly techniques discussed below are simply automated versions of the above process.
See this discussion on Last Paragraph issues.