The centrality of TV, and specific forms of TV at that, to Trump's rise. The way he effectively turned himself into a TV character. Into a symbol or an idea of something - in a way, a personification of the stereotypical image of a rich businessman. This touches on some postmodern ideas, especially the idea that in the modern mass media society, often, the image of something is perceived as more real than the original.
The early part of the book puts the focus on how Trump was born at a time when TV started to become a major force in US culture and society, so to some extent, you could say that Trump and the medium of television grew up together. The book claims that while, of course, as the son of a wealthy real-estate developer, he had to become a real-estate developer himself, at the same time, as a member of the first generation in the USA that grew up with TV, he ended up wanting to be a TV star, and a lot of his life was about trying to reach that goal, and eventually reaching it.
The book contrasts the TV and general media landscape of the mid-to-late 20th century with the one that started to grow in the 1980s and 1990s and took over in the 21st century. Under the old system, a few major organisations tried to sell TV shows to as many people as possible, and usually did that by being as pleasant and inoffensive as possible. There were some attempts to be a bit more daring, but they usually petered out after a while. In that environment, Trump was already able to become a regular presence on TV, but the more unpleasant sides of his personality meant that there were limits to how much of a star he could become.
Then came the much more fragmented media landscape created by cable TV and the internet, and, as a result of the breakdown of norms about what TV could show, reality TV. In this environment, people could do well for themselves by selling just to specific segments of the market, and Trump could become a superstar. Reality TV allowed him to turn his offensiveness into an asset and made him one of the most famous people on TV, and politically polarized cable news allowed him to become a major political figure, eventually - with additional help from Twitter - catapulting him into the White House.
In the book, this is presented together with commentary on how various kinds of media setups affect the collective psychology of society. That commentary seems to owe a lot to various "postmodern" or at least postmodern-influenced media theorists. In the last parts, there's emphasis on how people, as a result of perceiving the world through the media rather than directly, sometimes simply can't see a difference between facts and fiction any more.
Looks like the book has already influenced me to some extent - I just spent several paragraphs writing in a style and using vocabulary that I would probably have mocked a month ago.