there are tapes of some of the conversations Wolff had with White House staff (and Trump? Lordy, I hope there are tapes!). Also, Trump might want to reconsider his strategy, since the "cease and desist" huffing and puffing seems to have resulted in the book's release date being moved up and a general whipping-up of sales. (The excerpts continue to be amazing.)
But you know what is also amazing? The "obstruction of justice" case Mueller is shaping up regarding the President. It's also a view of a shambolic White House, and happens to be very well-sourced.