Air fare - post a new question in the "transportation" section. There are several fare-watch websites (other forum members are familiar with them). Consider Milan as an alternative gateway. It's a much bigger airport so there will be a lot more flight options. It might be better flying into Venice and out of Milan. It's about 2.5 hours by train between the cities, another hour from Milan to the airport. You want Malpensa, the main MIlan airport, not Linate. That would be an "open-jaw" flight, into one city and out of another. When looking for flights, you have the option of round-trip, one-way, or multiple destinations. Choose the last.
Hotels - Most hotels won't have Spring 2020 rates before fall, some may not have them until year-end. March is low season in Venice, so you shouldn't need to book more than 2-3 months out. In the meantime you can research hotels and narrow down your list, based on current prices. I'd pick dates in November, also low season, to get rates that are nearer to what you'll actually pay in March. Most reservations can be cancelled up to a few days before arrival. Be sure before you commit that the booking can be cancelled for free. Otherwise, they will charge your credit card.
As for your last question . . . . I would happily spend a week in Venice. But on a first trip to Italy, I'd want to see a little more than just one place. Venice is not the best base for day-tripping, since it could easily take 30-60 minutes to get from your hotel to the train station. Consider spending 2 nights somewhere else - which 2 and where depends on where you're flying out of. You didn't ask about trains, but you probably will start thinking about that too. If you buy tickets well in advance (5-6 months), you can get considerable discounts. For instance, from Venice to Milan, a full-price ticket is €50, but a super-economy is only €27.90. Venice to Verona is €28 or €14.90. (Double that for 2 people, double again for round-trip, adds up pretty quickly doesn't it?)
Which guide book? If it were me, I'd get the current version for planning hotels, day-trips, etc. Hotel and restaurant recommendations may change. The most important for hotels is choosing the area - that won't change. You should do your own online research (and come back here with questions too). Rates will change but I never depend on a guide book for that. With one or two possible exceptions, sights will be the same from year to year. You should always check online for entrance fees, hours, possible closures and temporary exhibitions anyway.