Trip to Rome Florence Venice

The store actually does a really good job of explaining what each book has. Check the "Is This the Right Book for Me?" section.

For RSV it says:
Does this Rome, Florence & Venice guidebook cover these three cities any differently than Rick's separate guidebooks for each city?

Yes: The separate Rick Steves guidebooks for Rome, Florence, and Venice offer more in-depth sightseeing information, several additional self-guided city walks and museum/sight tours, and more specifics on shopping and nightlife, plus suggestions for kids. The separate city guidebooks also include thorough coverage of several day-trip options for each city (and the Florence & Tuscany guidebook offers extensive coverage of the rest of Tuscany). Other information is generally the same, as this three-city book includes nearly all the hotels, restaurants, and nitty-gritty practical advice for each stop that you'll find in each city book. If you don't expect to be spending at least four days in each city, the three-city guidebook is all you need.

I would say it depends on how long you're going to be spending in each place. If you are going to do an really in depth visit the more specific guidebooks may have more detail you'll enjoy. But there is also the internet so if you have lodging and travel nailed down and don't plan on depending on Rick's narration for many museums or sights the Italy book is probably fine. The internet can help guide you to restaurants and through sites as well.

I believe the Italy book is also huge at this point. Note the Rick advocates pulling out the pertinent sections and only taking what you need.

Places like Rome has more sights than you can see in two weeks so you'll only be hitting the highlights anyway. After two of the big sights Venice is mostly about getting lost and admiring the city. Florence has a bunch of "must sees" for many people and so as long as you hit those most people are happy.

Hope that helps, have a great trip!
=Tod