Historiography on imperialism is pretty wide-ranging. Any specific interest?

As an example, one of the best books I read while going through comprehensive exams was Anne McClintock's Imperial Leather: Race, Gender, and Sexuality in the Colonial Contest. I definitely recommend it. But if you're looking for diplomatic or military approaches to imperial history, it might not be your thing.

Since you don't provide any qualifiers to your inquiry, I can at least provide some examples of the literature on American imperialism:

The Tragedy of American Diplomacy by William Appleman Williams

• Considered a revisionist examination of U.S. foreign policy, Williams illustrates how American imperialism, beginning with the Philippines, frequently ran counter to the traditional American mantra of democracy and self-determination.

Among Empires: American Ascendancy and Its Predecessors by Charles S. Maier

• Published in 2007, Maier's book situates the expansionism of American in the late nineteenth and early twentieth centuries within the larger narrative of global imperialism.

Fighting for American Manhood: How Gender Politics Provoked the Spanish-American and Philippine-American Wars by Kristin L. Hoganson

• This examines early American imperialism through the lens of gender, arguing that concepts of manhood impacted leaders like Teddy Roosevelt and led to calls for expansion through warfare.

Overthrow: America's Century of Regime Change from Hawaii to Iraq by Stephen Kinzer

• While Kinzer is not a historian, his book details many of the negative aspects of American foreign policy, specifically the willingness to alter and debase governments that did not align with America's global vision.

For the Soul of Mankind: The United States, the Soviet Union, and the Cold War by Melvyn Leffler

• Though technically not an examination of American imperialism, Leffler examines the domestic and global issues that confronted America during the Cold War. In my estimation this book remains one of the best works on the Cold War to date.

These books provide a decent overview of American imperialism, but there are entire historiographies on more specific subjects like Sino-American relations. There's also the upcoming Encyclopedia on American Imperialism and Expansionism that should serve as a decent catch-all for general inquiries. If you're interested, I can provide a more in-depth reading list regarding Sino-American relations, nuclear diplomacy, African Americans and foreign policy, and foreign policy during the 1960s.

I have a passive interest in the Congo, and know of a few books.

King Leopold's Ghost (1999) by Adam Hochschild

A wonderful introduction to imperialism in Belgian Congo. It's an overview of how King Leopold's personality, and character led effected acquisition and governing of the Congo, the eventual backlash it caused, as well as the "forgetting" (as Hochschild calls it) of the atrocities. Due bear in mind it is "popular history", so it's written as if it was a narrative, and you won't find the most in depth examination

Heart of Darkness by Joseph Conrad While not a history book, per say, I don't think one can study imperialism, at least in the Congo, without reading Heart of Darkness. Heart of Darkness is based mostly on Conrad's own experiences in the Congo and it brings up a lot of the implications of Imperialism and the atrocities committed in the Congo. If you plan to focus on the Congo, this is a must read.