Rick Steves' Italy book covers only the main visitor places. I'd start at your library and pick up as many guides as they have. After reading them you will have a better idea what you want to see. I then buy updated books that give me more information on the places I decided to visit. We traveled with both the Rick Steves book (because I like the amount of info and style of presentation on places he covers) and a Michelin Green Guide (because Rick leaves too many holes in his coverage of Italy).
One thing I think you should consider carefully is nights (rather than days). To have a full day somewhere, you need to plan a night on either side, so two nights equals one day, three nights equals two days, etc.
Between those stays will be travel days. I like to plan for no more than four hours travel time on a travel day because that day includes packing, checking out, getting food, getting to your train, making connections, getting food, getting off at the right stop, getting to your lodging, getting settled, getting food - and maybe some time for a look around. Invariably a travel day is a very full day. Out of necessity you may have to plan a longer travel leg, but realize it will be a really long day.
Other times, you may not need a full day. Pisa for example can be seen easily in four hours. Other than the Field of Miracles (where the tower and cathedral are located), there isn't much to see. If you can book an early train, give yourself time for a visit in Pisa, then continue on to CT, you can add that time back into your itinerary.
We chose to skip Pompeii and see Ostia Antica, just outside of Rome, instead - primarily to add the extra travel time back into our itinerary. I still plan to see Pompeii, but will include it with a route south of Rome (during winter months).