Final answer:
The provided reference material does not specify who the best-selling author of the 1990s was, despite discussing book sales and publishers.
Step-by-step explanation:
The best-selling author of the 1990s, who sold over 60 million copies of his books in that decade, isn't specified in the provided reference material. The information you've included about publishers and book sales is informative, but does not answer the student's question. The mentioned authors, such as President Bill Clinton, President Bush, and poet-novelist Dickey, are influential figures, but the documents do not confirm that any of these individuals was the best-selling author of the 1990s. Without the precise facts, I am unable to provide an accurate answer about who the best-selling author of the 1990s was.