kudos to the author:
Schwartz expressed regrets about his involvement in the book, and both he and the book's publisher, Howard Kaminsky, said that Trump had played no role in the actual writing of the book. Trump has given conflicting accounts on the question of authorship.[4]
Judging from the timing of him coming out to claim that (when Trump is running for presidency), it is very likely that he might have his own hidden agenda or could be doing it for the democrats.
It would be more believable if he had come out earlier.