This is the best English language bookshop in Rome. The owner Dermot O'Connell is a great source of information about the fantastic selection of books he stocks and will also be able to tell you where and where not to go. If you need a guide book, a good summer read, an Italian themed read or want to choose from his wide selection of non-fiction titles you must visit via del Moro 45. He also stocks some set books if you need something if you are studying in Rome. I love it!!!!
Via del Moro, 45, 00153 Rome, Italy
+39(0)6 5836942
Google map: bit.ly/pqIbbX
Updates
Have to disagree. If in Trastevere try instead Open Door (used books). Otherwise there's the Anglo American or the Lion Bookshop in the old "English Ghetto" of the Piazza di Spagna (and the German bookshop, Herder has some English books too). As for Italian bookshops: Fahrenheit 451, Libreria del cinema, Libreria del viaggiatore, 900 di carta, Simon Tanner ...
Is this your business?