If you're interested in Italian unification, I'd look at "Italy in the Nineteenth Century" by John A. Davis.

However, a lot has happened to Italy since then, including two world wars and a dictatorship. 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 where our twenty-years rule kicks in.