Here's a long The New Yorker article that goes into great detail about the genesis of the dossier.
Here is a 42 minute NPR interview with the author, Jane Mayer.
A lot of new facts are revealed. One interesting fact is that the DNC and Hillary didn't know who was working on the opposition research and Steele didn't know who he was working for. A DNC law firm hired Fusion GPS, and very few people knew about the connection.
An Excerpt from the New Yorker Piece.
In a normal political climate, the U.S. government’s announcement that a foreign power had attacked one of the two dominant parties in the midst of a Presidential election would have received enormous attention. But it was almost instantly buried by two other shocking news events. Thirty minutes after the statement was released, the Washington Post brought to light the “Access Hollywood” tape, in which Trump describes how his celebrity status had allowed him to “grab” women “by the pussy.” A few hours after that, WikiLeaks, evidently in an effort to bail out Trump by changing the subject, started posting the private e-mails of John Podesta, Clinton’s campaign chairman. The intelligence community’s assessment was barely noticed.
Steele finally met again with the F.B.I. in early October of 2016. This time, he went to Rome to speak with a team of agents, who avidly asked him for everything he had. The news generated by the publication of the D.N.C. e-mails had triggered the change. It had led the Australians to reconsider the importance of George Papadopoulos’s claims, and to alert American authorities. On July 31, 2016, the F.B.I. had launched a formal investigation.
The agents asked Steele about Papadopoulos, and he said that he hadn’t heard anything about him. After the meeting, Steele told Simpson that the Bureau had been amassing “other intelligence” about Russia’s scheme. As Simpson later told the Senate Judiciary Committee, F.B.I. agents now “believed Chris’s information might be credible.” Although the Bureau had paid Steele for past work, he was not paid for his help on the Trump investigation. Orbis remained under contract to Fusion, and Steele helped the F.B.I. voluntarily. (He did request compensation for travelling to Rome, but he never received any.)
Soon after the meeting in Rome, the F.B.I. successfully petitioned the Foreign Intelligence Surveillance Court for a warrant to spy on Carter Page. Trump’s defenders have accused the Bureau of relying on politically motivated smears to spy on Trump’s campaign, but by then Page was no longer an adviser to Trump, and the F.B.I. had collected information in addition to what had been supplied by Steele.
The Bureau encouraged Steele to send any relevant information he came across, and that October he passed on a questionable item—a bit of amateur sleuthing that had been done by someone he’d never met, a former journalist and self-styled investigator named Cody Shearer. Jonathan Winer, Steele’s friend at the State Department, had shared with him an unfinished memo written by Shearer. Not only did it claim that the F.S.B. had incriminating videotapes of Trump having sex in Moscow; it also made wild allegations that leaders of former Soviet states had given huge payments to Trump family members. Steele wasn’t aware that Shearer had longtime ties to the Clintons, as did Sidney Blumenthal, a Clinton ally, who had given Shearer’s report to Winer. Steele had never met Blumenthal, either, but he dutifully jotted down the chain of custody on the cover of the report before sending it on to the F.B.I., with the caveat that he couldn’t vouch for its credibility. He noted, though, that some of the findings were “remarkably similar” to Orbis’s.
Trump’s defenders have seized on the Shearer memo, which Steele didn’t write, using it to argue that Steele’s research was politically tainted by the Clintons. Sean Hannity’s official Web site carried the inaccurate headline “christopher steele authored another dossier, used clinton contacts.”
As the election approached, the relationship between Steele and the F.B.I. grew increasingly tense. He couldn’t understand why the government wasn’t publicizing Trump’s ties to Russia. He was anguished that the American voting public remained in the dark. Steele confided in a longtime friend at the Justice Department, an Associate Deputy Attorney General, Bruce Ohr (whose wife, Nellie Ohr, was briefly a contractor for Fusion). In a memo to the F.B.I., Bruce Ohr recalled Steele saying that, given what he had discovered, he “was desperate that Donald Trump not get elected and was passionate about him not being President.” According to people familiar with the matter, Ohr and other officials urged Steele not to be so upset about the F.B.I.’s secrecy, assuring him that, in the U.S., potentially prejudicial investigations of political figures were always kept quiet, especially when an election was imminent.
Steele was therefore shocked when, on October 28, 2016, Comey sent a letter to congressional leaders: the F.B.I. had come across new e-mails bearing on its previously closed investigation into Hillary Clinton’s use of a private server as Secretary of State. He said that these e-mails required immediate review. The announcement plunged Clinton’s campaign into chaos. Two days before the election, Comey made a second announcement, clearing her of wrongdoing, but by that point her campaign’s momentum had stalled.
To Steele, the F.B.I., by making an incriminating statement so close to Election Day, seemed to be breaking a rule that he’d been told was inviolable. And, given what he—and very few others—knew about the F.B.I.’s Trump investigation, it also seemed that the Bureau had one standard for Clinton and another for her opponent. “Chris was concerned that something was happening at the F.B.I.,” Simpson later told the House Intelligence Committee. “We were very concerned that the information that we had about the Russians trying to interfere in the election was going to be covered up.” Simpson and Steele thought that “it would only be fair if the world knew that both candidates were under investigation.”