The topic you are interested in is a very vaste one get started by clicking the following link http://en.wikipedia.org/wiki/History_of_Italy having read it all, you will know if you have had enough or you want to deepen the subject

Before somebody suggests it, definitely not David Gilmour's "The Pursuit of Italy". The book is overly-biased and the author himself not only appears to have an axe to grind against the very concept of Italy but also goes to incredible lenghts in mocking, belittling, underestimating and generally utterly dismissing whatever has been achieved in the past 150 years... and in doing so, he's doesn't disdain to repeat old stereotypes and outdated notions.

Literally "a book to be thrown away with great force".

On the other hand, Indro Montanelli and Roberto Gervaso's "La Storia d'Italia" really is a comprehensive, not to mention fairly accurate, overlook of our history from the Middle Ages to the 1990s; but I don't know whether there's an English translation out, though. Another good read - despite its flaws, as I personally found the author to have a slightly biased POV - would be Denis Mack Smith's I Savoia Re d'Italia ("Italy and its Monarchy").

Can you read Italian?