I'm in chapter 18 of 25 of Fire and Fury. The NYMag piece:
...is not really a summary of the book, but is basically the juiciest bits from the first two/three chapters. Think of the book as more of the same, going chronologically through the runup and his presidency so far.
Wolff's book is almost tangentially about Trump: he shares plenty of anecdata painting Trump as the toddler that the other three (Jarvanka, Bannon and Priebus) mightily try to lure to their envisioned kind of presidency. Bannon with the nihilistic, apocalyptical view of the world; Jarvanka with an almost Democrat view of making the world better and fixing the Middle East, and Priebus with the Republican view. He also says that Bannon's place in this is pretty much because of Mercer, so it makes sense that it also ends with Mercer.
If the Russia investigation is Stupid Watergate, Wolff's book is Stupid House of Cards.