I am totally clued out, and would like to read some good novels (historical or modern) before or during my trip in order to get a better sense of the places I will see. Thanks very much!

There's a book called The City of Falling Angels by John Berendt. It's set in historical Venice - see below.
Venice, a city steeped in a thousand years of history, art and architecture, teeters in precarious balance between endurance and decay. Its architectural treasures crumble—foundations shift, marble ornaments fall—even as efforts to preserve them are underway. The City of Falling Angels opens on the evening of January 29, 1996, when a dramatic fire destroys the historic Fenice opera house. The loss of the Fenice, where five of Verdi's operas premiered, is a catastrophe for Venetians. Arriving in Venice three days after the fire, Berendt becomes a kind of detective—inquiring into the nature of life in this remarkable museum-city—while gradually revealing the truth about the fire.
Hope that helps. You'll love Venice, it's beautiful.

Sarah Dunant has written several historical fictions about Florence and Venice. I especially enjoyed Mapping the Edge.

Hi there,
Definately don't read "under the tuscan sun" as all my Tuscan friends say they can't stand that author and the way she portrays Italy.
I took an italian literature class and I was struck by the book La Storia by Elsa Morante. It's a very engaging and thought provoking novel about italy after WWII. If you're into history and want to understand contemporary italy this is a good course book!
The first half of "Room with a View" by EM Forrester is set in Florence about a young English woman on holiday.
Agree about "City of Falling Angels"... he does a great job of contrasting today's Venice with its past.