purchasing guidebooks in Rome

Try a Feltrinelli book store. Some of their locations carry RS guidebooks. If the one you visit doesn't have them, ask at the information counter as another location may have them in stock. Their store near Piazza della Repubblica would be a good place to start.

Or you can go online, where you are now, and download the kindle or other e-reader versions....

I agree with Nigel, easier than spending sightseeing time locating a book, you can buy an e-book.

La Feltrinelli at Largo Argentina should have at least the Rome book. You might have to buy the Paris one in Paris.