Never underestimate the treasure trove that is your local used bookstore. While browsing idly yesterday, I came across the book Affreschi: Exploring Etruria, the only book I know that focuses on the once Etruscan domain that stretched from Rome to southern Tuscany and east to Umbria. Profiled, for example, are the towns of Civitavecchia, Cosa and Tarquinia, as well as the villages around Lakes Bracciano, Bolsena and Vico.
My copy, published in Italy in 2001, is unfortunately out of print. But apparently a revised edition came out in fall 2006. So, if you’re interested in ancient Etruscan culture or unique day trips from Rome, this may be a great supplement to a general guidebook. Information is available on the website Elegant Etruria. Also check out the Italo Shop for more specialty guides to Italy.