It's more explicitly about Belgian colonialism, but many of the featured figures in King Leopold's Ghost by Adam Hochschild are British or Anglo-American (including Henry Morton Stanley of "Dr. Livingston, I presume?" fame). Hochschild is a journalist, but the book is historically rigorous and generally well-regarded by historians. There is one misstep in his treatment of the personal life of one of his figures (I found it overly salacious and irrelevant to the story he was telling), but otherwise the book is a compelling (and gut-churning) read.

ninja edit for a typo