Trattorias of Rome, Florence, and Venice
This 274-page paperback, which will fit in a large jacket pocket or smallish handbag, contains listings in the three title cities of places to find traditional food and drink, sometimes in highly evolved form. The establishments range from medium-raunchy up to quite elegant, and thus cover a good deal more than simply trattorias, but that wouldn't all fit in the title.
Besides the listings, which try to take some notice of cultural, even archaeological, context, there's a glossary and lots of etiquette tips and practical advice. I love telling people how to behave in Italian restaurants.
People tell me they enjoy reading the book even when they're not looking for a place to eat. I do think it gives value beyond what you can download free from the Internet.