‘Long Mile Home’ by Scott Helman and Jenna Russell

Globe reporters Scott Helman and Jenna Russell present an exhaustive narrative of the Boston Marathon bombings, the search for the terrorists, and the city’s recovery.Dina Rudick/globe staff

Three days after the bombs exploded at the finish line of the Boston Marathon, President Obama spoke to the wounded city from the Cathedral of the Holy Cross, in the South End.

“It was part of the job,” Scott Helman and Jenna Russell write in “The Long Mile Home,’’ “this solemn presidential ritual of showing up where disaster strikes — to the hamlet on the plains flattened by a tornado, to the small town swallowed by biblical flooding, to the suburb rocked by a mass shooting.”

And then, a year after those tornadoes and shootings, comes the publishing ritual of a book to recount the whole harrowing experience. It is a curious sub-genre, these anniversary books, as reportage and craft are subservient to a tight schedule, authority less important than expediency.

It is fortunate, then, that the ritual chronicling of the marathon bombing fell to Helman and Russell. Accomplished journalists, they are identified on the cover of “Long Mile Home’’ in type only marginally smaller than their own names, as “Reporters for The Boston Globe.”

That seems less recognition of their credentials than a reminder of the resources available to them: an entire newsroom that just won a Pulitzer Prize for its coverage of the bombing. And they have used those resources to full effect, producing by the far the most exhaustive account to date of April 15, 2013, and the months that followed.

It is perhaps not the definitive story of why two brothers blew up a marathon, if only because there are still too many unknowns (involving Russian and US intelligence, a triple murder in Waltham, an FBI shooting death in Florida, and various smaller questions about decisions by local law enforcement officials during the investigation, pursuit, and capture of Tamerlan and Dzhokhar Tsarnaev) and a year simply isn’t enough time to write that version anyway.

But “Long Mile Home’’ — at times gripping, occasionally hopeful, always heartbreaking — is essential to understanding what happened last April, and, in some ways, the city in which those events happened.

If daily journalism is the first draft of history — and the mechanics of the modern news cycle actually reduce it to a rough sketch — then a book like this is the second, carefully curated and highly distilled. In the moment, in the relentless deluge of tweets and cable-news chyrons and digital headlines, perspective is the first casualty. Facts and semifacts and outright nonfacts move too quickly, each receding the instant the next pushes in.

The value of “Long Mile Home’’ is that it reprocesses that unwieldy mess into a coherent narrative, one that pulls a full, clear story from among thousands of characters and millions of anecdotes.

Helman and Russell tell that story mostly through five people — a police officer, two victims, a surgeon, and the marathon’s surprisingly interesting race director — with a supporting cast of dozens.

There are no headline revelations, and it’s unclear to a casual reader even how much material is wholly new, but that hardly matters: The randomness of the destruction, the way mere inches determined who was maimed or killed, is freshly stunning; a carjacking three nights after is harrowing; and the fluky discovery of Dzhokhar Tsarnaev hiding in a wintered-over boat is a scene as taut and precise as any high-end crime fiction.

Indeed, Helman and Russell have a fine ear for detail, often to lovely effect. When Boylston Street ceased being an active crime scene and reopened to traffic, “Two panhandlers got back to work at the corner of Exeter Street” — a wonderfully gentle hint of the city returning to normal.

They could have benefited from a finer filter for those details — knowing the governor found his favorite Thai place on Yelp is a step too far — and a sharper editing pen would have excised the occasional noir swagger (“Besides, there wasn’t time for competition. There were terrorists to hunt down.”).

But those are quibbles. “Long Mile Home’’ isn’t a literary narrative, nor is it intended to be. It is meant to bear journalistic witness, and at that is succeeds admirably.

Helman and Russell for the most part maintain their reporters’ distance, but their book is also in part a tribute to the city, their city, so they can be forgiven a lapse in tone now and again. “In an era of social and political fragmentation,” they write of the moments after Dzhokhar Tsarnaev was captured, “it was perhaps the closest Boston would come to a shared, unifying moment.”

They have memorialized that moment, and much of what led to it and followed, with a thorough and graceful chronicle.

■  Correction:  Because of a reporting error, an earlier version of this review incorrectly stated the location of a triple homicide to which Marathon bombing suspect Tamerlan Tsarnaev has been linked.