Another line of inquiry into Trump’s business deals unearthed a flow of Russian money into his projects, what Fritsch once called a “tour de sleaze.” Needing a clearer sense of what was happening inside Russia itself, where public records were hard to come by, Fusion reached out to Steele.

Image Peter Fritsch, co-founder of the private intelligence firm Fusion GPS. Credit... Peter Fritsch

The authors chronicle how Steele became so alarmed by what his sources were telling him that he asked Fusion’s permission to share his raw intelligence notes with the F.B.I. and, later, an adviser to Sen. John McCain.

Steele, Simpson and Fritsch started talking on deep background to journalists, too, though the authors say they took care not to share the dossier with the media before the election, and were furious when BuzzFeed posted the document in January 2017, 10 days before President Trump’s inauguration. (They show little love for The Times and its 2016 election coverage, either.) This timeline, they repeatedly argue, is key: Republicans have tried to portray the dossier as a hoax or a dirty trick designed to prejudice the electorate, but how could it have swayed voters if it was kept hidden before the vote?

Simpson and Fritsch are able guides to a byzantine world; their presentation is methodical, almost lawyerly, which isn’t as bad as it sounds. When reading a story full of weird financial transactions, narratives and counternarratives, it’s helpful to have everything laid out as plainly as possible — even if the layers of chicanery are sometimes so densely packed that their syntax gets squeezed into ugly shapes. “The story described how a former senator from Putin’s political party who had gone on to run the Central Bank of the Russian Federation was the subject of an investigation in Spain into money laundering by a Russian organized crime syndicate called the Taganskaya Gang,” they write, describing a news article; it’s a sentence only the most grimly determined reader could love.

Simpson and Fritsch try to address conservative conspiracy theorists head on, devoting an entire chapter to their work with a Russian real-estate company named Prevezon and its lawyer Natalia Veselnitskaya — who, unbeknownst to Fusion at the time, arranged a notorious meeting with the Trump campaign. For a couple of guys who spent their careers investigating how money can shape incentives, or at least appear to, they seemed for a while either defensive or naïve when it came to the murkier aspects of their own business model.

Fusion’s conservative critics doubtless won’t be placated by this book, even though the authors say that those critics were ultimately what made the book possible. Only when Republican members of Congress forced Fusion to provide documents and testimony in an attempt to ferret out a vast left-wing conspiracy were Simpson and Fritsch freed to write about interactions they “would have otherwise been contractually obligated to keep confidential.”

It’s a nice bit of irony in a book that reads like a morality tale about unintended consequences. As Simpson told congressional investigators back in 2017: “We threw a line in the water and Moby Dick came back.”