Complex sentence: Schwartz expressed regrets about his involvement and asserted that Trump had played no role in the actual writing of the book, claiming that it was highly doubtful that that Trump had ever read a book in his adult life, let alone written one.
Split into simpler sentences: Schwartz expressed regrets about his involvement and asserted that Trump had played no role in the actual writing of the book. Schwartz also expressed doubts that Trump had ever read a book in his adult life, let alone written one.