Nearly all the books I have read about the internet have deepened my fears about the net effect of social media on the health of our body politic. For example, I thought three facts from the congressman Ro Khanna’s recent book, Dignity in a Digital Age, were enough to scare anyone concerned about the future of democracy.

Khanna reported that an internal discussion at Facebook revealed that “64% of all extremist group joins are due to our recommendations”; he revealed that before 2020, “QAnon groups developed millions of followers as Facebook’s algorithm encouraged people to join based on their profiles”; and he pointed to a United Nations report that Facebook played a “determining role” in events in Myanmar that led to the murder of at least 25,000 Rohingya Muslims and the displacement of 700,000 others.

Seen and Unseen, a brilliant new book by Marc Lamont Hill, a Black professor, and Todd Brewster, a white journalist, certainly doesn’t ignore those dangers. But the authors’ focus is overwhelmingly on the positive effects of Twitter and Black Twitter, which they argue have democratized access to information, and the power of the smartphone to provide the incontrovertible video evidence needed to prosecute the murderers of men like George Floyd and Ahmaud Arbery.

The book is a brisk, smart, short history of the effects of new communication technologies, from the photographs of the 19th century to the movies and television of the 20th and the internet of our own time.

It includes terrific mini-portraits of many of the heroes and several of the villains of the Black-and-white battle which has dominated so much of American history, including the great Black abolitionist, Frederick Douglass, who turns out to be the most photographed American of the 19th century, and the white supremacist Thomas Dixon Jr, whose novel The Clansman was the basis for the 1915 film The Birth of a Nation.

There is a great section about the impact of The Birth of the Nation, which single-handedly revived the Ku Klux Klan and did more to rewrite the history of Reconstruction than any other book or movie. Its director, DW Griffith, was frank about wanting to give white southerners “a way of striking back”.

“One could not find the sufferings of our family and our friends – the dreadful poverty and hardships during the war and for many years after – in the Yankee-written histories we read in school,” Griffith wrote. “From all this was born a burning determination to tell … our side of the story to the world.”

As the authors note: “His movie did that spectacularly.”

The book also reminds us that this was the first movie shown in the White House and the host, Woodrow Wilson, was a friend and Johns Hopkins classmate of Thomas Dixon Jr. Wilson, of course, was also the president who allowed the segregation of the federal government.

But what makes this volume especially valuable is the authors’ capacity to see the good and the bad in almost everything.

WEB Du Bois said The Birth of the Nation represented “the Negro” either “as an ignorant fool, a vicious rapist, a venal or unscrupulous politician, or a faithful but doddering idiot”. James Baldwin called it “an elaborate justification of mass murder”.

And yet the film was so egregious it also had a tremendous positive effect – it “did more to advance the NAACP”, which had been founded six years earlier, “than anything else to that date. In essence it jump-started the movement for civil rights.” At that time, that term did not yet have any meaning.

Du Bois and the NAACP hoped to hit back “in kind” with a movie called Lincoln’s Dream but were stymied by “the lack of enthusiasm” of white capital.

In our own time, Hill and Brewster identify the unique power of the video of the murder of George Floyd, which “resonated with whites because the cruelty inflicted on him was so undeniable, so elemental … and so protracted (nine minutes 29 seconds) that it could be neither ignored nor dismissed”.

For Black people of course it was much more personal: as they watched “the last breaths being squeezed from Floyd’s body, they could see themselves in his suffering; or an uncle, or a sister, or even a long-departed ancestor”.

A beautiful mini-biography of James Baldwin includes many of his most pungent observations, including, “Not everything that is faced can be changed; but nothing can be changed until it is faced.” And, “To be a Negro in this country, and to be relatively conscious, is to be in a rage almost all the time”.

It turns out that “one of the most frequently cited BLM counterpublic voices is Baldwin’s”. He is “the movement’s literary touchstone, conscience, and pinup” as well as its “most tweeted literary authority”.

That is the most positive contribution of Twitter – and particularly Black Twitter – I have ever heard of.

The authors write that Baldwin “was impatient with America because he saw it as trapped in its own history”, and wanted America to admit “that it owed its very existence to an ideology of white supremacy”.

There was a time in my life when I considered that an exaggeration. But once you have acknowledged that ours is a nation that was literally founded on genocide and slavery, Baldwin’s judgment becomes an indisputable truth.

• Seen and Unseen: Technology, Social Media, and the Fight for Racial Justice is published in the US by Atria Books