The Two Princesses of Bamarre by Gail Carson Levine, 2001

This book tells the story of two sisters: Meryl, the elder who is brave, and Addie, the younger who is not brave at all. But when Meryl gets stricken with the Grey Death, Addie knows she must go searching for the cure. In her journey, she confronts ogres, specters, gryphons, and a dragon. She finds the courage within to try to save her sister. Slow-going at the beginning.