Which Rick Steves book?

Take a look at the descriptions of the different books under "shop online" on this website. They are notably candid for a site that's selling you stuff, hopefully giving you useful advice about which one to buy.

On your other thread I suggested that, rather than take day trips from Venice, you spend the time in Venice that the city deserves (including nearby islands) and then move your base to one of the mainland cities, like Padua, that interest you. Given how many places you want to see, you might just invest in the "Italy" book, which will be useful if and when you visit other parts of this wonderful country.

I agree with Dick about possibly splitting you time. For one thing, hotels in Venice are probably going to be a lot more expensive than in other cities. At the very least, I'd want to pay close attention to the location of my Venice hotel vis-à-vis the train station.

The "Best of" books are less in-depth than the others; I think they have color pictures, don't they? This website will tell you whether the "Venice" book has more details on Venice than the "Italy" book does, which I assume is the case.

Since you have some time, how about getting the books (prior years) from the public library, and comparing them for yourself to see which formats appeal to you.

To learn about how a book compares with other similar ones, go to its listing on this website under Shop Online, then click the "Is This The Right Book For Me?" tab.

Here's the listing for Rick Steves Italy: https://store.ricksteves.com/shop/p/italy-guidebook

In general, the Best of guides have less text and information, and more pictures (they're printed on heavier paper that allows for nice color pictures). I don't personally see the appeal of that, but I assume Rick, after all these years, knows his market better than I do; if people didn't want to buy these kind of guides, he wouldn't have started selling them.