I'd like to recommend Tamora Pierce's books, especially the Alanna the Lioness quartet and the Protector of the Small quartet. Both are about lady knights in the medieval world of Tortall. Another good series if you like medieval-set police procedurals is the Beka Cooper series - Terrier, Bloodhound, and the newest is Mastiff, which comes out in a couple of weeks.

Talk about girl power, these books have it.

Roxy