I have not read the book but from the excerpts I have seen there is nothing that will surprise Trump’s critics or alter the idolization by his fans. I’m glad Bob Woodward is keeping himself busy and staying relevant, but this book isn’t going to change anything, and Trump boosters aren’t even going to bother reading it. This book gets a few days in the news cycle pundit discussion circuit before going into the bargain bin.
As for Trump’s support, while there has been some erosion by the middle class and conservative Hispanic voters to put him in office (and farmers in generally cannot be too happy with the impact of his tariffs even with a $12B subsidy), overall his base has not reduced that much since the initial drop in early 2017, and in fact has increased this year from its lows in mid- to late-2017. That base was never that large to begin with, and shouldn’t have guaranteed him an electoral victory in 2016, so whether or not he will be re-elected depends far more on how well the Democratic party does of selecting a viable candidate, and the candidate does of selling a message that appeals to non-Trump-base voters, but it isn’t as if coal miners, iron workers, and evangelical Christians are going to abandon Trump en masse.
The impact on mid-term and 2020 Congressional elections is still unclear; the Republicans have created their own problems more or less independent of Trump in not being able to advance their promised agenda. But the extreme polarization of politics and the adverse influence of social media means that there are a lot of voters who will not abandon the GOP to vote for “the enemy” even if it is clear that Republican lawmakers are just rearranging deck chairs. The optimism for how things are going to radically change because of how bad a job the current band of scoundrels is doing should be tempered by the fact that this is analogous in many ways to Nixon and the GOP circa 1973, and they came back just fine after that Carter fellow proved to be just too darned nice.