Rebecca Walker's memoir "Black White and Jewish"

You're looking for Point of Sale data, which publishers use for all sorts of things, including fundamental business decisions (I know this because I used to load and report on that data for Hachette Book Group). It is (mostly) obtained from Nielsen Bookscan. Nielsen does not provide it for free as that is a part of how Nielsen (the same as the TV ratings people) makes its money.

However, that having been said, you can do some checking directly on Amazon, Wal-Mart, Costco (if the book is sold there), Barnes & Noble, Waldenbooks and other retailers, both online and brick and mortar. Plus, you can check with the publisher, Riverhead Trade (a division of Penguin), but they will most likely only reveal this data to the author or the author's agent.