During a stint with the C.I.A., Mr. Harding writes, Mr. Snowden appears to have grown increasingly disillusioned with the tactics of the United States government. He joined the N.S.A. as a contract employee in 2009; over the next years he said he discovered how far-reaching the N.S.A.’s surveillance activities were — “they are intent on making every conversation and every form of behavior in the world known to them.” He also became skeptical of Congressional oversight of the agency.

Mr. Harding provides some bright digital snapshots of Mr. Snowden’s time in Hawaii, where he worked as an N.S.A. contractor. He describes Mr. Snowden’s seemingly idyllic life on Oahu, with his girlfriend, a dancer and photographer named Lindsay Mills. Some of their friends teasingly likened Mr. Snowden to the vampire character Edward Cullen in the “Twilight” series because of his pale, solemn appearance, Mr. Harding reports. “E,” as Ms. Mills reportedly called him, occasionally made an appearance in her photos online, Mr. Harding writes, but his face was routinely concealed.

Tension builds as Mr. Snowden settles on a plan — to leak top-secret stolen documents to journalists interested in civil liberties — that, Mr. Harding writes portentously, would “almost certainly” result in his “incarceration for a very long time and possibly for the rest of his life.” And there is more suspense as Mr. Snowden engineers a high-risk rendezvous with Mr. Greenwald, Ms. Poitras and The Guardian’s veteran Washington correspondent Ewen MacAskill in a Hong Kong hotel room. There, the journalists vet their source, and Mr. Snowden begins to talk them through the trove of incredibly complex and technical N.S.A. material — all while anxiously fearing a knock at the door.

Mr. Harding gives us a spy-novel-like sense of the tradecraft that Mr. Snowden and the journalists used, from encryption to old-school huggermugger meetings, but some mysterious gaps in the Hong Kong-to-Moscow story line remain. Of the period in Hong Kong after Mr. Snowden left that hotel, Mr. Harding writes vaguely that Mr. Snowden “went underground” with the help of a “mystery guardian angel” — “a well-connected Hong Kong resident” — who seems to have found him a place to stay with a friend. At one point, he writes, Ms. Poitras called Mr. MacAskill with the alarming news that Mr. Snowden had sent a message saying he was in danger. Hours later, a lawyer phoned back to say Mr. Snowden was O.K. “The details were hazy,” Mr. Harding writes, “but it appeared Snowden had survived a close call.”

As for Mr. Snowden’s ability to protect his information while living in Moscow, Mr. Harding writes: “Snowden was extremely good at digital self-defense. When he was employed by the C.I.A. and N.S.A., one of his jobs was to teach U.S. national security officials and C.I.A. employees how to protect their data in high-threat digital environments.” Paradoxically, he adds, Mr. Snowden now “found himself in precisely the kind of hostile environment he had lectured on, surrounded by agents from a foreign intelligence agency.”

After The Guardian published its first scoops, there was a surreal scene in which, Mr. Harding reports, two men from GCHQ, “Ian” and “Chris” — called “the hobbits” by staff members at The Guardian — arrived at the newspaper’s London office to oversee the destruction of Snowden data, directing the demolition of computer parts with power tools and a data-erasing device. According to a November 2013 article by The Guardian’s editor, Alan Rusbridger, in The New York Review of Books, British government officials insisted on this destruction, despite explanations from The Guardian that such efforts were fruitless because copies of the Snowden material by then existed in other countries.

Mr. Harding — the author with David Leigh of “WikiLeaks: Inside Julian Assange’s War on Secrecy” — spends considerable space in these pages chronicling goings-on within The Guardian’s offices in London and New York, and explains the legal difficulties British journalists face without the sort of protections provided by the United States Constitution. Unfortunately, such discussions of the trans-Atlantic differences in journalism occasionally devolve into parochial and dubious boasts of British superiority, with Mr. Harding going so far as to assert that American newspapers “pursue leads at a leisurely, even gentlemanly pace,” and that the American news media generally act “deferential towards the president.”

Portions of “The Snowden Files” seem particularly aimed at a British audience, focusing at length on the surveillance activities of the GCHQ and its eager-to-please relationship with its wealthy American counterpart. But the book still gives readers, who have not been following the Snowden story closely, a succinct overview of the momentous events of the past year. And if it leans toward dramatizing everything in thrillerlike terms, the book also manages to leave readers with an acute understanding of the serious issues involved: the N.S.A.’s surveillance activities and voluminous collection of data, and the consequences that this sifting of bigger and bigger haystacks for tiny needles has had on the public and its right to privacy.