Comey's memoir "A Higher Loyalty" seems to (inadvertently) vindicate Christopher Steele's and Glenn Simpson's perception that the FBI did not take Steele's warning seriously.
"A Higher Loyalty" describes the events that led up to the Comey Letter. As Secretary of State, Hillary Clinton used a private email server to conduct government business, discussed classified materials over unsecured email, and deleted some emails that should have been preserved. This showed poor judgment, but the way the law is written, these are not crimes if there was no criminal intent. Comey explains that Clinton, a typical person of her generation, simply did not understand the technology and never intended to break the law. Under the law, the FBI had no basis to prosecute her.
The situation changed a few weeks before the election when FBI agents stumbled upon a laptop that contained some of the emails they had believed deleted. Comey struggled with what to do next. Should he inform Congress that the investigation into Clinton's emails had been re-opened, or should he keep the investigation quiet? If he kept silent, he explains, the FBI might discover incriminating evidence against Clinton after she was elected President, and the Bureau would be falsely accused of suppressing that information to help Clinton.
Comey decided the most important thing was to prevent the FBI from showing bias, and he informed Congress. This backfired spectacularly. In his clumsy attempt to avoid showing favoritism to Clinton, Comey showed favoritism to Trump instead. Comey's Letter generated so much negative publicity for Clinton that she lost the election. Comey's second letter exonerating her came too late.
Comey attempts to accurately record his tenure as FBI Director, but his book is telling in what he does not say. In the months leading up to the 2016 election, then-candidate Trump broke campaign finance laws by soliciting contributions from foreigners. Trump's inner circle had dozens of contacts with Putin's regime at the same time that Putin's hackers were committing crimes in the United States. Trump bent over backwards to deny Putin's cyberattacks were happening and ridiculed anyone who reported them. While all this was happening, investigator Christopher Steele warned the FBI that Putin might have compromising material on Trump that could be used for blackmail. The FBI Director's memoir never mentions any of this. If Comey was even aware of it -- which does not appear to be the case -- he never orders an investigation.
Comey does not mention the Steele Dossier until describing his first meeting with Trump after the election. Simpson later told Congress that Steele tried to warn the FBI about Trump's links to Putin, and no one listened. Comey's book appears to confirm this.
Ironically, Comey was so focused on investigating Clinton -- who had not broken the letter of the law -- that he ignored Trump, who had. Moreover, Trump had publically announced that, if elected, he would order torture, order the murder of terrorists' relatives, deport innocent American citizens, and discriminate against minorities. All those things are illegal.
Comey saw the FBI's mission as upholding the law and staying out of politics. Comey does not seem to have realized that by announcing that the Clinton email investigation had been re-opened, he wasn't keeping the FBI out of politics. Instead, he directly inserted the FBI into politics. Doing so helped elect Trump, who had repeatedly stated he had no intention of upholding the law.