It's hard to know what to post on here: D for ThirD, I for III, or 3. I'll go for I
I Promessi Sposi
(The Betrothed) Alessandro Manzoni
It's a classic in it's way, thought of as the greatest Italian novel, and groundbreaking in being written in ordinary Italian. I dunno about greatest, but it still reads very well (in English, I don't know Italian).