Michelle Obama's memoir has sold nearly 10 million copies

The candid memoir, released in November, quickly became a best-seller and is one of the most popular books of the decade.

"We believe this could be the most successful memoir in history," said Thomas Rabe, the chief executive of Bertelsmann, a parent company of Penguin Random House, according to the Wall Street Journal.

Obama has sold out arenas on her massive international book tour, where she has been interviewed onstage by Oprah Winfrey, Sarah Jessica Parker, Reese Witherspoon and former Obama senior adviser Valerie Jarrett, among others.

In its first week, "Becoming" sold more than 1.4 million copies, Penguin Random House said in November.

The book had the longest streak at No. 1 for any book since "Fifty Shades of Grey" came out in 2012, according to Amazon.