"Harry Potter and the Deathly Hallows"
Author: J.K. Rowling
Hardcover/Paperback Sold: 44 million
We're not going to spoil the ending if you haven't read the last Harry Potter title, in which Harry confronts He-Who-Must-Not-Be-Named. When it went on sale in 2007, the book sold 15 million copies in the first 24 hours [source: Forbes].