There have been various articles written summarizing some of the things in Mary Trump's book.
https://www.politico.com/news/2020/07/07/nine-details-donald-trump-niece-book-350808
This one has a top 10 list. None of it is earth shattering. #6 on the list is something I've long believed to be crucial to understanding very poorly chosen one's dysfunction. His mother almost died giving birth to his younger brother and spent a long time recuperating. Even after she recovered his friends have described her as a distant presence. It is odd that he now favors a hairdo similar to hers. His relationship with his father has been written about extensively, but imo it is his relationship or lack thereof with his mother that is more important. Take that for what it's worth from a not-very-good amateur psychologist.
6. Trump's personality is the product of his relationship with his mother
The author suggests that Donald Trump’s personality is shaped by the weak relationship she says he had as a child with his mother, also named Mary Trump. Because of that, the president’s niece suggests that Donald Trump turned as a child to his father, who was also not a warm parent.
“Donald suffered deprivations that would scar him for life” and developed personality traits such as “displays of narcissism, bullying [and] grandiosity” as a result, the author writes.