Among the very best guides to Italy are the ones in the Time Out series. These are published in Britain and have up-to-date tourist-type info, but also significant cultural and historical material. All in a pleasing, well-illustrated layout. They'll run you about about 20 USD. As much as I know about Venice and the Veneto, I always like to have the latest edition of their guide for the area.
Other guides in the series covering Italy are Florence, Turin, Rome, Naples, and Milan. The guides for Venice, Florence, and Naples also cover the most noteworthy adjoining areas and towns.