If I have the Italy 2013 book on my Kindle, is there any way to get just the 2018 update without purchasing an entire new book?
Nope, not that I’m aware of. New edition for 2019 debuts on September 18th.
Trouble is, there's not an "update", just a new book with new/updated information sprinkled through it. What you CAN get, free, is the listing of updates on this site. (See "guidebook updates" from the home page.)
The list of updates will give you those changes judged critical enough to mention online, but of course not all the little details that changed over the course of 5 different editions.
My advice is that's not the place to try to save money. Pay the $19 and have everything in one place.
"My advice is that's not the place to try to save money. Pay the $19 and have everything in one place."
That's my advice too. And no, I don't work for Rick and I don't get a cut of his sales. But think of the time and money you'll waste if you follow old advice that is no longer true (closed restaurants, moved bus stops, places that didn't need reservations in 2013 but do now, etc).
I agree with Rick that good guidebooks are the key to a good trip, and for what a trip costs, "saving money" by not buying a current guidebook is a false economy. Of course, it's relative; I might try to use one from 2017, but since yours is 2013 it's just not worth using it.