The Eragon Advanced Readers Copy is an uncorrected proof that was released to editors and reviewers ahead of the Knopf Hard Cover release in August 2003. Due to this being an unofficial release, the book has no ISBN.  These ARCs are designed to drive up hype for books before release by giving reviewers early access to books. “Eragon was distributed to more than 3000 reviewers and Library Association, According to Knopf’s director of publicity” - John Bankston

 Eldest would also get an ARC, however, Christopher didn't like the idea of his work being released uncorrected. By the time Brisingr came around he had enough say to stop any ARCs from being made.