Here is some advice I will pass on to people who are just beginning writing:
When you start writing your book, just write in one document (although make sure you frequently make backup copies of it on more than one device!). If you find yourself deleting things early (which you generally should avoid doing, but, hey, it happens), cut the section and paste it into another document and save it under a title that reasonably explains what’s in it. Sometimes things I cut out of my first book wind up, in some form or fashion, in my sequels. And, of course, there’s always the possibility that you will want to put something back in that you cut out. (That happened with an entire chapter that was cut from my first draft, but was put back in the second draft.)
Once you more or less have your book completed, and it comes time to do the editing, start saving your book as a different file every time you work on it. For instance, I have “Book Two 081511.” When I save changes that I make to it today, I will do a “Save As” and save it as “Book Two 081811.” I have found this is easier to keep up with than copying and saving every paragraph and sentence that I end up deleting. If I end up needing to find something I’ve cut, I can just pick an earlier version and go to the appropriate chapter. I can also search Word using a word or phrase which I remember being fairly unique to the portion I removed.