During my nine years of living in Rome and devouring every available nook and cranny of its layer on layer of civilization, I came to realize that were I to live there twenty years, I would never see it all; that there was always something new to discover. The profusely illustrated DK Guide to Rome proves my point, making me ask "How could I possibly have missed that?!".
The guide to Rome begins with a timeline and brief history that covers about 2700 years. Particularly interesting are the cutaway illustrations, for instance of an aqueduct, or the Baths of Diocletian (in which one can still get a sense of the original by walking into the gigantic church of Santa Maria degli Angeli e Martiri). The book also explains the elements of architecture, the floor plans of some of Rome's 410 churches; it then cites the most accessible museums; and fountains and Egyptian obelisks. The rest of the book is divided into regions, such as the Capitol, the Forum, the Palatine, etc, and it includes nine guided walks and brief descriptions of day trips in the environs of Rome, such as Hadrian's Villa. The Guide is so comprehensive, in fact, that one could not possibly see all of its recommended museums, archaeological sites, and churches unless one were going to stay at least a year.
I love DK guides for their elegance, their stunning illustrations, and their attention to detail. Because of their sheer weight, however, they are not the least bit practical to take along, and will very quickly seem like excess baggage. So study this one before you go, and when you come home, enjoy the places you have seen and look forward to those you have yet to see on your next journey of discovery.