To be clear, Pillar of Fire and its prequel Parting the Waters document short-run nonviolent triumph over the forces of segregation: children pushing through firehoses in Birmingham and marches to the Selma voting registrar's office. The civil rights movement's discipline yields political rewards. The political party most allied to them, the Democrats, win an epic landslide in Congress and the White House. The Civil Rights Act of 1964 passes by a wide margin, to be followed by wide margins for the Voting Rights Act of 1965 and Lyndon Johnson's Great Society programs, which line up with Martin Luther King, Jr.'s increasing concern with poverty.
Most strikingly, politicians begin to define their identities with respect to racial problems. Of course, politicians' taking on the fight against racism and defining their identities around it is a good thing, and Lyndon Johnson makes the matter a personal crusade. This is paired, though, with many politicians' taking the other side in the fight: opposing the civil rights movement, and doing so vocally. Southern politicians' growing support for the Republican party is explicitly built around common opposition to federal civil rights action. In short, the debate is polarized, and it is clear in the book where this polarization is headed (see the Southern strategy).
Until now I'd always seen civil rights protests of the 1960s as irrefutably effective. The concrete institutional changes they seemingly achieved ended explicit discrimination and legal segregation, and there are many signs of progress, including a sharp decline in hate crimes and rise in life expectancy. Those changes, though, do not necessarily prove the movement effective because we do not know what would have happened without civil rights demonstrations. In the 1960 election, there was ambiguity as to which party would be more supportive of civil rights: both parties made overtures to civil rights advocates. One can imagine a slower process ending in the same legal changes occurring. It seems clearer from Pillar of Fire that the drama and conflict of civil rights confrontations played a significant role in triggering a white backlash that led to the war on drugs, mass incarceration, and residential and educational segregation on a par with pre-civil rights America.
To be clear, I still think it would be the wrong conclusion to take from the historical record that civil rights era protests led to more backlash than success. At the very least, though, I'm significantly less bullish on actions I'd previously thought to be clearly effective.