Sorry J.K. Rowling! You've been beaten fair and square by Suzanne Collins on Amazon.com.
“To achieve this result in just four years is a great testament to both the popularity of the work and, we think, the growth in reading digitally during that time. Customers love these books and all three titles are consistently on our Top 10 lists in both print and Kindle formats, and The Hunger Games is also the most-borrowed book in the Kindle Owners’ Lending Library.”
With two more blockbuster movies still to come, we don't see the sales dying down any time soon!