Daniel Immerwahr is not an outwardly provocative guy. He is slender and mild, from suburban Philadelphia, comes off serious, but not arrogant, smartly appointed, but not flashy. His office in the history department at Northwestern University is standard issue, rectangular, lined with books, a few personal artifacts, but nothing striking. He has a tattoo on his right arm of the Charles Demuth painting “I Saw the Figure 5 in Gold.” Otherwise: a casting director’s dream of a young, unpretentious history professor.

But there are different ways to be a history professor, he says, waving a hand as if to introduce each. “There are different modes. One is outrage — the tragic history of what modernity has wrought. Some go for antiquarian — ‘How different and weird the past was!’ But the feeling I most prize as a historian is ‘Holy (expletive)! I didn’t know that!’”

Take “How to Hide an Empire,” his new book.

At a glance, it looks like a dive into the history of American territorial (yawn) expansion. And it is, a deeply researched, often revelatory reframing of history as seen through the islands, prairies and military bases that the United States has claimed as its own. It is about how “mainland” Americans came to regard — or perhaps, disregard — places like Puerto Rico and the Northern Mariana Islands, along with the U.S. citizens in those territories. It is about, as Immerwahr describes, the nation as a “pointillist empire.”