In Print:

Rick Steves' Rome 2006, Rick Steves

Fodor's Rome, 6th Edition, Robert Fisher

Rome:  Eyewitness Travel Guides, DK Publishing

Lonely Planet Rome:  City Guides, Duncan Garwood

Rough Guide to Rome, 2nd Edition, Martin Dunford

National Geographic Traveler:  Rome, Michael Brouse

Frommers Rome, Darwin Porter

If you can find it, Wonders of Rome, probably at a library, still one of the very best, originally written by Fattorusso in 1956 and reprinted in 1968

The portion dealing with Rome in Mark Twain's Innocents Abroad

On the Web:

Schmap 2006 Rome Guide  (free digital guide with maps, photos and listings)

Rome.info  (online travel guide)

Romaturismo.com  (official Rome travel website)