Benedict Anderson, R.I.P., Imagined Communities, one of the most influential books on the topic.

There are books about nationalism in every country you can think of. For American nationalism, I'd recommend Anatol Lieven's "An Anatomy of American Nationalism."

I wrote an assignment whilst at university on the why nationalism has been important in history since 1789, these books helped me massively; Benedict Anderson, Imagined Communities, Michael Billig, Banal Nationalism, Anthony H. Birch, Nationalism and National Integration, John Breuilly, Nationalism and the State, Karl Deutsch, Nationalism and Its Alternatives, E. J. Hobsbawn, Nations and Nationalism Since 1780, John Hutchinson & Anthony D.Smith, Nationalism, Vincent P. Pecora, Nations and Identities, Sukamar Periwal, Notions of Nationalism,

As above, Anderson's Imagined Communities and also Hobsbawn's Nations and Nationalism since 1780 were probably the most useful