Tamora Pierce

Tamora Pierce writes fantasy novels set in lands where knights still ride by and people fight with swords and sorcery. However there is a twist, the heroes are girls! In worlds dominated by men, these teenage (and sometimes younger/older) girls have to find out what their gifts are and where they fit in. Her books divide into two distinct universes, Tortall and the Circle-verse. Tortall is a medieval kingdom where the eldest sons become knights of the realm, until Alanna of Trebond decides she wants to be a knight and swaps places with her brother Thom who goes off to train as a sorceror. She has to keep her real identity a secret and only the family man-at-arms who accompanies her knows the truth, at least to start with! Alanna's story comprises the Lioness Rampant quartet, Pierce's first series, and Alanna crops up again in The Immortals quartet and The Protector of the Small quartet as well as in the two books concerning her own daughter, Trickster's Choice & Trickster's Queen. Not all of the books are set entirely in Tortall, they move around it's neighbours too, allowing the characters to interact with a variety of people, some of the fun comes from working out which country the fictional ones are most alike! The latest Tortall books are set 200 years before Alanna sets out to win her shield and concern a very different life to that of the nobility, Beka Cooper is a member of the Provost's Guard (a sort of medieval police force) who learns to use her unique skills to bring down criminals and write wrongs in Terrier and Bloodhound, the third in the series is hopefully due out in the next year or so.
The Circle-verse is a different place, with very different characters. Instead of one central protagonist, this series of 10 books (and counting) revolve around 4 young people and their burgeoning magical gifts. Briar is a street rat destined for the cells, Daja and outcast whose people blame her for the deaths of her family, Triss has been passed around her family because they're afraid of the strange things that happen when she loses her temper and Sandry is a lady from one of the most powerful families around but just wants to spend her time sewing. Together these four have the power to halt earthquakes, bind winds and save those they love. They just have to learn to work together, and their teachers at Winding Circle temple will attempt to show them how.
All of her books are great slices of fantastical fiction with heart. They're perfect for a teenager struggling to find her place in the world and who needs to know she can rescue herself, she doesn't need to wait for a prince!
Check out www.tamora-pierce.com or her livejournal http://tammypierce.livejournal.com for more info.