Tamora Pierce is an American YA novelist. She has written several series, but her best known series is The Song Of The Lioness. Oddly, it began as one long novel, aimed at adults, but her agent suggested that it might work better, and sell better, as four books for young adults. It did. Her books have been translated into twenty languages and won several awards.
The Song Of The Lioness series is set in a place called Tortall, a mediaeval-like kingdom where women don’t get to be knights. But Alanna, the heroine, wants to become a knight. Her twin brother, Thom, wants to be a sorcerer. Alanna was supposed to travel to a temple in the City of the Gods and Thom was to go to the palace for training as a knight.
The siblings decide to swap places so they can do what they really want to do. Alanna disguises herself as a boy and goes to train for knighthood as a page called Alan, while Thom goes to the City of the Gods where he can learn to be a sorcerer.
Alanna is able to keep her identity a secret. One thing I liked about the story is that she has to deal with things like having a period. Too many stories with disguised girls don’t take that into account.
In the course of the series, she grows up, becomes “the lady knight” and marries.
The Immortals series is set in the same universe as Song Of The Lioness, with another heroine, and the adult Alanna appears in it. The heroine Daine is gifted with Wild Magic and can talk to animals, heal them and even shapeshift to animal form.
By this time, women in Tortall can become knights and practise other occupations they couldn’t in the start of the Lioness series, so there are a lot of strong, interesting female characters.
There are several other series, which have all done well, but these are the ones I have enjoyed the most.
All of her books are available in ebook and print.