Catherine, Called Birdy by Karen Cushman
But she lives in the thirteenth century, and already her father is planning her marriage. Catherine has no intention of succumbing to a life of domesticity with any of the boors, popinjays or dolts her father considers a good match, but her ruses to repel the parade of suitors cannot succeed forever, however often she picks her nose in front of them.
Karen Cushman doesn't downplay the unpleasant realities of medieval life. The manor is dirty, flee infested and freezing in winter. Public executions are a form of entertainment. Romantic love is a luxury few can afford and women are property, their purpose to increase their family's prosperity. But despite this, Catherine will not be browbeaten, and her courage and spirit as she grasps friendship and finds adventure despite the restrictions imposed on her make her a worthy heroine.
Feminist fans of historical fiction written in diary form rejoice (I know you're out there, it can't be just me). This book's got your name on it.