1. Robert Bellah, "Civil Religion in America" (1967). Not a book- just an article. I'd start here to get some ideas rolling on American nationalism. It's a classic, influential essay arguing that despite one's chosen religion, Americans embrace a national set of beliefs and ideals, forming a "civil religion."
2. Benedict Anderson's Imagined Communities (reprinted 1991): An older reference, but quite influential. You'll find his definition of nationalism within the first 10 pgs, and he formulates a social constructivist view of nationalism. I'd start here for a definition of nationalism to use, critique, or develop your own ideas against.
3. Bruce Lincoln's Holy Terrors: Thinking about Religion after September 11 (2006): Not a book on nationalism per se, but definately relevant to American nationalism in the G.W. Bush years. I would recommend this book if you want to examine how political rhetoric and religious discourse (whether chosen religion or civil religion) influences nationalist sentiments. (A quick look at ch.2, "Symmetric Dualisms" will help determine if you want to use this book, as it quickly outlines two conflicting perspectives, implying that nationalist sentiments are at times defined against opposing ideals).
-Good luck on your search!