The most popular modern history of Italy (in English) was written by the British historian Paul Ginsborg, however some Italianists criticize him for being a little too dismissive of the admittedly convoluted Italian political system. You can find it under the title "A History of Contemporary Italy, 1943- 1988." Ginsborg ends his history with the fall of the "Pentaparty" governing coalition in the 1980s, because the 1988-1992 legislature was primarily characterized by a series of drastic changes to the country's political system. Mark Gilbert, another British historian, picks up from more or less that spot and is somewhat more attentive to the social aspect of the country, he's written "The Italian Revolution" which rather conveniently ends more or less right where our twenty-years rule kicks in.

If you're interested in why Italy is a country in the first place, you'd have to look at "Italy in the Nineteenth Century" by John A. Davis in the Short Oxford History of Italy (a series which is itself, in my opinion, the best general history of Italy).

But if, as you claim, you know absolutely nothing about Italy and would like to have a more general (perhaps lighthearted) picture of its society and people, Luigi Barzini's "The Italians" and the more recent "La Bella Figura" by Beppe Servegnini are two places to start for a humorous contemporary overview of the country explicitly written by Italians for foreigners (the first book, although one of my favorites, is from the 1960s, the other is more recent and from the early aughts).