Trekkerbp, on each guidebook page there's a section "Is this the right book for me?" It will answer your questions better than we can. Here's a sample, from the Italy 2020 page:
Is This the Right Book for Me?
What's the difference between this Italy guidebook and Rick's Best of
Italy guidebook?
Rick Steves Italy, the complete guide, offers more coverage of more
destinations in a mostly black-and-white format, and is updated
annually.
The shorter Rick Steves Best of Italy guidebook focuses on Italy's top
destinations and sights, is in full color, and is updated every other
year.
Both books are packed with hand-picked recommendations and Rick's
travel advice and tips for saving time, money, and hassle.
Should I get the Sicily guidebook in addition to this Italy guidebook?
Yes! If your trip includes Sicily, you'll also want to bring Rick
Steves Sicily, as it's the only Rick Steves guidebook with any
information on Sicily.
Should I get the Rome, Venice, and/or Florence & Tuscany guidebooks
(or any of the Pocket guides) in addition to the complete Italy
guidebook?
If you're not spending more than a few days in any major city, Rick
Steves Italy is all you're likely to need.
If, however, you're spending four days or more in the same city, the
extra information in a single-city/region guidebook can we worthwhile.
Compared to the corresponding chapters of the Rick Steves Italy
guidebook, our full-size guidebooks for Rome, Venice, and Florence &
Tuscany offer more in-depth sightseeing information, several
additional self-guided city walks and museum/sight tours, more detail
on day-trip options, and more specifics on shopping and nightlife,
plus activities for kids. Other information is generally the same —
for example, the Rome chapter of Rick Steves Italy has nearly all the
hotels, restaurants, and nitty-gritty practical advice for Rome that
you'll find in the Rick Steves Rome guidebook. Likewise, Rick Steves
Italy includes coverage of most of the same Tuscan towns (except
Cortona), and the same practical advice and hotel and restaurant
listings that you'll find in Rick Steves Florence & Tuscany. Rick
Steves Florence & Tuscany, however, has more in-depth sightseeing
information for Florence, Siena, and Pisa, as well as extra
information on Florence's shopping, nightlife, and kids' activities.
It would make less sense to supplement Rick Steves Italy with one of
the smaller Pocket guides for Rome, Venice, or Florence. Each does
have one or two walks and museum tours that aren't included in the
related chapters of Rick Steves Italy, but otherwise they offer far
more limited coverage of each city's sights, hotels, and restaurants.
(Our Pocket guide to the Cinque Terre, however, has no more advice —
except for its foldout map of the region — beyond what you'll find in
the Cinque Terre and Italian Riviera chapters of Rick Steves Italy.)
Should I get any of the Snapshot guides for Italy in addition to the
Italy guidebook?
No, because the various Snapshot guides for Italy are simply excerpted
chapters taken directly from the complete Rick Steves Italy guidebook.