Is it worth getting a newer guidebook?

I wouldn't bother with a new book if it was only 1 year old, but in 12 years a lot has changed regarding transportation and certainly any hotel or cafe recommendations.
And for sure the maps are better!
It's only $25 dollars. You'll be glad you you have it.

I think it just depends on how you use one--fares for entry, opening times, and that sort of thing can change, but most of that is verifiable on the web. I rarely use hotel or restaurant sections, but the basic background section will be unchanged.
CT has certainly changed quite a bit since 2005; for example, landslides have closed the "easy" trail.

I found a 2015 on ebay for $3.74. Gonna splurge, lol!

The "newest" RS Italy Guidebook is the 2018 model. Italy may be "timeless", but 13 years is a long time.

Try your local public Library. It's free, don't ya know?

Calculate the cost of a new book as a percentage of the total cost of your trip. A rounding error at best. Buy the new one!

Another good source for a recent ( occasionally even new) RS books is thriftbooks.com

Information relates to maximizing trip enjoyment. Me? Buy it.

Now that is a false savings unless you are only using it for site descriptions.

I got our Italy 2017 book on Amazon. It was a good deal using our Prime account.

You should be find with a 2015 guide, as long as you verify critical information online.

I don't get all the effort going into saving maybe 15 bucks compared to the overall cost of a vacation. But it's your time and effort.

I love getting a new guidebook and marking it up with my notes. Then I tear out the sections I need for my trip. I feel it is a small price to pay to improve my travels.

I have a 2006 Italy book. Before my trip this past February, I bought the 2016 book. The old book has about 750 pp, the new one, over 1250. The increase is mainly because a lot more towns are in the later books - I didn't find that sightseeing for places like Rome had been noticeably expanded and since I'd spent a good bit of time there in the past, the new book wasn't much help. There was little change in things like self-guided walks for the places I was planning to visit. While much of Italy is centuries-, if not millenia-, old, there are always sights that are renovated or opened for the first time. Of course, recommendations for tours, guides, rooms and food are much changed.

One thing I didn't like about the new book is that it's about the same thickness as the old one - because it's printed on very thin paper (like a bible). When I tried to photocopy pages, the print on the reverse side bled through, as did highlighting. If I had to do it again, I'd buy the ebook.

take it out of the local library; the branch here has usually more than one publisher and new editions.

The 2015 edition is a rather pointless compromise, in my book, but you can see some of the most recent news summarized at https://www.ricksteves.com/europe/italy/guidebook-updates.

Consider the total coast of your trip vs. the minuscule cost of a new guidebook. The new book may help you avoid mistakes and end up saving you money and aggravation.

We buy the new book every time we go to Italy. It's part of the excitement of planning. Also, you will find new things in the new book, and some things that have been removed. The 2017 book doesn't have info on Cortona, but the 2013 does. We buy the new, look at our planned itinerary to compare, and sometimes take two books with us. RS is usually so right on point and you never know where the day may lead you. Worth the few bucks in my opinion.