Limited Special Collector's Ultimate Edition: Each book has received a "Deluxe Edition" reprinting which usually includes some goodies like drawings from cover artist John Jude Palancar and/or Christopher Paolini himself. Deluxe Editions are usually released a year after the original publication and can be distinguished by a ring of gold around the cover. Eragon and Eldest were reprinted together in an Omnibus edition which was so big it could be used as a bludgeon.
Shrug of God: If anything even remotely related to a theory for an upcoming book is mentioned to Paolini's face or asked in an interview, he always responds with "No comment."
What Could Have Been: Christopher Paolini originally intended Eragon to have simply blessed Elva as a baby, but found out after the publication of Eragon that he had made some grammatical mistakes himself in creating the ancient language, resulting in Eragon's wording being "may you be a shield from misfortune". Rather than simply correct the error, he decided to make it a plot point in future books.
Word of God: Christopher Paolini answers a lot of questions about the Cycle, mostly in interviews. Just look at all the examples throughout the pages for this series.
Write Who You Know: The behaviors and mannerisms of Angela the herbalist were based largely on Christopher Paolini's sister, Angela.