Final answer:
The specific best-selling author of the 1990s is not mentioned, but authors like J.K. Rowling, John Grisham, and Danielle Steel were highly successful. Advances for prominent figures' memoirs can reach millions of dollars, as seen with President Bill Clinton's memoir.
Step-by-step explanation:
The best-selling author of the 1990s who had more than 60 million copies of their books sold during that decade is not specified in the question. However, several authors achieved significant sales during that period. Notably, authors like J.K. Rowling with the Harry Potter series, John Grisham with his legal thrillers, and Danielle Steel with her romance novels were incredibly popular. Authors can receive substantial advances for their books, with famous figures sometimes receiving millions of dollars to write their memoirs. For example, President Bill Clinton received a $10 million advance for his memoirs, indicating the high value placed on notable literary works.