Catherine Called Birdy by Karen Cushman
Catherine is a medieval teenage girl born centuries before her time and Catherine Called Birdy is her diary. Unlike the romanticized accounts of history, Catherine keeps track of the real stuff, including flea bites and privvy usage. She is slightly tomboy, and longing to become a modern woman, although such a thing won't exist for about 800 years.
Catherine's father has decided that he wants to marry her off, but Birdy maintains veto power, if only by behaving so disgracefully that she runs off her potential suitors. She even takes to nicknaming them the likes of Shaggy Beard and Master Lack Wit.
Despite the "way back when" setting, this book comes off pretty timeless. Apparently, teenage girls have been the same for millenia. For all the things that have changed (marriage at 14, indoor plumbing, dowries) the essence of a girl growing up doesn't change. She clashes with her parents, can't decide whether she wants to be a child or a grown up, wants more control of her life, has fits of temper that she later regrets, gets involved in things out of her maturity level, just like every other adolescent, probably ever. I love that Cushman paints a picture of a girl who even modern girls can relate to, and reminds us just how hard life could be in the 13th century.