Warbreaker

Brandon Sanderson's Warbreaker (Tor, 2009) follows two Idrian princesses, Vivenna and Siri, after a treaty sends one of them to marry the God King of the rival nation of Hallandren. Published as a standalone novel, it shares a world with, and has characters who later appear in, the Stormlight Archive. This series groups the novel with its two-part split editions.