It kind of depends on why you're hoping to retain the old book.
Technically, when you paste over it with the new version, you're still retaining the old version. It will permanently be in your history--so all you have to do is go to the "Compare Projects" tool, pick the date when you replaced the old text, and you should be able to compare the new text with the old text. This has the added advantage of both 1) not requiring any extra setup, and 2) allowing you to compare each verse, new vs. old. It has the disadvantage that the "old" version isn't completely obvious to a new person--they would have to know to look back into the history for Ruth.
Another option is simply to go to the My Paratext Projects folder for your project, copy the 08RUT???.SFM book out, and save it anywhere you want.
And, yes, copy/pasting it into a XX book is possible.