In Manchild in the Promised Land, does the author speak more about "black power" or civil rights?
To the extent that "black power" and self-empowerment are the same, I'd say he's more concerned with black power. In the Harlem that Brown describes, civil rights, i.e. equal protection of rights under law, don't mean all that much. Mostly, his story is "beating the street" through education, the importance of which he realizes after being imprisoned for theft. The world he lives in is hostile, and whites are definitely complicit in making it that way, but he's concerned with how to get himself out of it. This is made clear when his brother slides into a life of crime.
Certainly, the book is not written as a rejection of Martin Luther King's wing of the civil rights movement, which is how the black power movement is often interpreted, but it's definitely presenting a very different side to that struggle. Brown faces poverty, drug addiction, crime, and lack of opportunity, none of which were magically fixed by the actions of civil rights leaders and politicians. He had to overcome them by himself, and he did.