Stephenie Meyer (pictured) sold over 15 million books in the US in 2008, and that's just in outlets tracked by Nielsen Bookscan, where various editions of her books claimed 9 of the top 50 spots for the year.
That total sale is millions of copies more than JK Rowling's tracked sales from books in the top 50 last year, when Harry Potter and the Deathly Hallows tracked 7,740,000, and two other titles on the list gave her sales of 8.6 million copies. (NB, that doesn't include other backlist editions that did not sell enough to make the top 50.)
Meyer occupies 6 of the top 15 slots, as books for children (at least in part) dominated the top of the list. Christopher Paolini and JK Rowling both made the top ten, and two of Jeff Kinney's "wimpy kid" books made the top 20.