I did not hear the NPR interview, but I have read _King Leopold's
Ghost_ by Adam Hochschild.  While Hochschild credits Twain with
sparking his own interest in Leopold's rule of the Congo -- Twain
receives only minor mentions in several places throughout the book.

_King Leopold's Ghost_ offers excellent insights into the history
of the Congo but more emphasis is given to characters such as Henry
Morton Stanley and Joseph Conrad than Mark Twain. A more thorough
discussion of Twain's involvement with E. D. Morel, the former dock
worker, and the Congo Reform Movement (and a source that is referenced by
Hochschild) is Hunt Hawkins article titled "Mark Twain's Involvement with
the Congo Reform Movement: 'A Fury of Generous Indignation' " published in
The New England Quarterly (June 1978); pp. 147-175.  Hawkins discusses
Twain's involvement with the movement and how he later became
disillusioned after discovering he had been mislead regarding the US's
involvement in the 1884 Berlin Conference which sanctioned Leopold's