Photographer: John Lamparski/WireImage
I haven't read Hillary Clinton's new book, "What Happened" (reviewed nicely by David Weigel here), but I can give an answer to what happened to her presidential campaign. Two answers, in fact.
On the one hand: Everything. When a candidate loses by such a close margin, absolutely everything could have made the difference. Had Clinton pulled resources from states that didn't wind up close and applied them to Pennsylvania, Wisconsin, and Michigan; had Clinton handled the email controversy a little bit better; had she run better ads; had Taylor Swift endorsed her. Even most of the things that political science hasn't been able to show make any difference at all might have been enough. Take, for example, early ads. Political scientists have found that the effects of ads fades rapidly, so that it's impossible to demonstrate that ads run weeks (or months or years) before an election have any eventual effect on vote choice at all. But the methods used are unable to distinguish very small effects from zero, and (quite sensibly) the norm is to treat anything which doesn't have a sufficiently demonstrated effect as if there's no effect at all. And yet when we're talking about extremely narrow margins such as the 2000 and 2016 presidential elections, it's quite possible — I'd go so far as to say likely — that some of those not-proven effects really did matter.
That's the context in which to assess claims about James Comey's investigation, especially the choices he made at the very end of the election, and of the press coverage of that investigation. In an election in which minor candidate gaffes or, for all we know, such unlikely suspects as billboards and lawn signs might have been enough to make a the difference, then of course Comey's letter to Congress about the investigation of a Clinton aide's laptop computer and the subsequent over-the-top press frenzy about it likely cost Clinton the election. And of course the media's year-long obsession with the email story at the expense of anything else Clinton said or did — along with a press strategy by the candidate and her campaign which failed to convince the media to cover her popular policy ideas — cost her the election.