NEW YORK (AP) -- Christopher Paolini's million-selling "Inheritance" fantasy series, originally planned as a trilogy, is being extended to four books, publisher Alfred A. Knopf announced Tuesday.

The first two books, "Eragon" and "Eldest," have sold more than 12 million copies worldwide. The third book, currently untitled, will come out next fall.

"I plotted out the 'Inheritance' series as a trilogy nine years ago, when I was 15. At that time, I never imagined I'd write all three books, much less that they would be published," Paolini said in a statement.

"When I finally delved into Book Three, it soon became obvious that the remainder of the story was far too big to fit in one volume. ... In order to be true to my characters and to address all of the plot points and unanswered questions Eragon and Eldest raised, I needed to split the end of the series into two books."

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