Eyewitness Travel Guides have lots of photos, floorplans, and maps, and suggestions about what to see, where to go, and what to do in Rome.
National Geographic Traveler's book includes photos and detailed maps, mapped walking tours, and visitor information.
Quiet Corners of Rome by David Downie explores 60 peaceful places of beauty in Rome, away from the noise and crowds of the city, each with beautiful photos. The book is small and easy to carry along on your visit to Rome. It's also a great gift book or book for the armchair traveler.
Rome: Dream of Italy introduces you to the best of the eternal city in a mobile app that's economical and easy to carry around. Edited by Kathy McCabe of Dream of Italy Travel Newsletter, this app includes entries written by a group of Rome experts covering places to go, eat, and stay as well as some hidden gems.
Flavors of Rome takes a look at the food of Rome and how it will be different from Italian food in the US. There are good restaurant recommendations and a handy food glossary at the end. The book is small and lightweight so easy to carry on a trip to Rome. Also available as an e-book.
Rome the Second Time, part of the Curious Traveler Series, has 15 itineraries that don't go to the Colosseum. If you've been to Rome before and want to see something more than the usual tourist sites, this book has detailed suggestions.
Lonely Planet has more than 800 places to go in Rome and 30 maps. There are local tips from residents of Rome and information about history, art and architecture, and where to eat and drink.
Small and lightweight, this book makes an easy companion guide to carry with you while traveling around Rome. There are nine chapters covering different neighborhoods of Rome and one on daytrips. It has good maps and a small section of Italian words, mainly food. I've used the older version on several trips to Rome.
Frommer's complete guide to Rome includes sights, walking tours, accommodations and restaurants, Italian phrases, and a pull-out map.
Rome is the largest archaeological site in the world. If you plan on visiting many ancient Rome sites, this is a great companion book. The book covers 12 main archaeological areas in central Rome and four in outlying areas and includes maps and diagrams.