Heartless by Anne Elisabeth Stengl

Princess Una of Parumvir has come of age and will soon be married. She dreams of a handsome and charming prince, but when the first suitor arrives, she finds him stodgy and boring. Prince Aethelbald from the mysterious land of Farthest shore has traveled far to prove his love and also to bring hushed warnings of danger. A dragon is rumored to be approaching Parumvir. Una, smitten instead with a more dashing prince, refuses Aethelbald's offer and ignores his warnings. Soon the Dragon King himself is in Parumvir, and Una, in giving her heart away unwisely, finds herself in grave danger. Only those courageous enough to risk everything have a hope of fighting off this advancing evil.

Series: Tales of Goldstone Wood