Clinton signed over a thousand copies of the book, Living History (search), at a promotional event at a Barnes & Noble store in midtown Manhattan Monday, the first day the book was on sale.
The company said the former first lady's White House memoirs sold more than 40,000 copies in the first 24 hours it was available, instantly making it an in-house best seller.
Nationwide sales figures for other booksellers were not immediately available.
Late Monday, publisher Simon & Schuster, which paid $8 million for the tome, announced it would print an additional 100,000 copies, on top of an extraordinary initial printing of 1 million copies.
Foreign rights to Clinton's book have already been sold in over a dozen countries.
Clinton's lawyer, Robert Barnett, said he also expected international sales to be "extremely high."