"Alanna, The First Adventure" is the first in the "Song of the Lioness" series by Tamora Pierce. It's about a girl who wants to be a knight in a mythical world where the gods are magical. Her father wants to send her to a convent to become a lady. He wants to send her brother to become a knight, but the brother wants to receive the magical training the convent provides boys. So the kids trade places so each of their dreams can come true. But being a knight turns out to be a lot more difficult than she had expected.
This story seems geared more toward girls because it follows Alanna's story, not really her brother's story. The book is probably good for readers ages 9 and older. Some adults may also enjoy it. It's not really appropriate for younger children because there's fighting and name calling.
I would recommend this book to adventurous girls who aren't in a hurry. It's not a quick read and there are some pretty big words. If you were just getting into chapter books, you probably wouldn't understand most of it.
Since the book was first published in 1983, the cover art has varied. So make sure you're spelling the title correctly when you type it in the library or internet search tool. You can read more about the book at the author's website here. Here are some of the covers, but go to her website if you want to see them all: