This book reads very much like a book-length newspaper article. For one thing, it's very timely: just three weeks after its publication, it has already started to disappear under a tide of new Trump-related news. By April, it will likely be hopelessly archaic, of interest only to Trump historians.
Secondly, Wolff collects information gathered from various sources. The result reads like a newspaper series, one too long and dense to fit in a single issue.
And finally, this book is what Wikipedia would call original research: Wolff has, apparently, collected information from some large number of interviews, and this book represents his attempt to organize them into some sort of coherent narrative.

On one hand, it's unfortunate that Wolff doesn't name his sources, or cite them in a way that allows us to check his facts. I hope that information will come to light in the future that either corroborates or debunks his facts, because as it stands, the credibility of the book is simply Wolff's credibility.
On the other hand, I didn't notice any huge discrepancies between what Wolff describes and what's widely known. That is, we may not have independent confirmation of what happened in Bedminster in August 2017, but we do know that Trump visited Bedminster at that time. So while Wolff undoubtedly got many of the details wrong, he's probably right about the big picture.

And the big picture doesn't hold any earth-shattering surprises: Wolff describes a candidate Trump who didn't think he would win, a transition that was chaotic because no one in the campaign planned for it because no one really believed Trump would win: they were supposed to get media coverage from the campaign that could be parlayed into consulting or lobbying jobs, the launch of a TV network, that sort of thing.
Finally, Wolff describes a White House in chaos because of the various actors and forces pulling and pushing each way, each with a different agenda. If you've been following the news, none of this is particularly surprising, though it is interesting to read the particulars.

The bottom line is, if you want to see what the buzz is (or was) about, read or at least skim this book. If you want to know what's going on in the White House, follow the news, and wait for future independent researchers to confirm or debunk Wolff's claims.