Brisingr - The Inheritance Cycle #3 by Christopher Paolini

Oaths sworn . . . loyalties tested . . . forces collide... Eragon is the greatest hope to rid the land of tyranny. Can this once simple farm boy unite the rebel forces and defeat the king? Following the colossal battle against the Empire's warriors, Eragon and his dragon, Saphira, have narrowly escaped with their lives. Still, there is more adventure at hand for the Rider and his dragon, as Eragon finds himself bound by a tangle of promises he may not be able to keep.

When unrest claims the rebels and danger strikes from every corner, Eragon must make choices-choices that will take him across the Empire and beyond, choices that may lead to unimagined sacrifice.