I'm fast becoming an expert on guide books to Rome. So far, my favourite is "City Secrets: Rome". I like it because it lists the things to see and places to eat followed by a short anecdote by someone who knows the place well, and explains why they recommend it in real, human terms.
For example, this is what Virginia L. Bush says about the Colosseum:
"A new visitor to Rome should go first to the Colosseum. Since it is said that Rome will stand as long as the Colosseum stands, and the world will last as long as Rome stands, it would be good to check first that everything is in order with the universe"