Alanna of Trebond doesn't want to go to a convent and learn magic, as per her father's wishes, she wants to become a female knight. Her brother Thom has no aptitude for fighting, and is better at magic than her, so when she proposes that she disguise herself as a boy, and they swap places, he has no complaints. Alanna convinces their man-at-arms to go along with the plan, and arrives in the capital as Alan of Trebond. She begins training as a page at the royal court, and makes both friends and enemies within her first few weeks there.
Among her friends, Alanna can count George, King of the Thieves, and Jonathan, the heir to the throne. None of them know her secret, but are impressed with how hard young Alan trains at fighting and scholarship. She gets through several years without anyone discovering her true identity, but when Jonathan becomes gravely ill, she may have to risk everyone finding out in order to heal him.
It's a great book, with a strong, determined and loyal protagonist, who risks her reputation to achieve her dreams of becoming a female knight. She's smaller and weaker than her fellow pages, and therefore has to work twice as hard to prove herself worthy. She's bullied by older, stronger boys, but refuses to let her friends fight for her, and figures out a solution on her own. When she breaks her right arm, she teaches herself to fight almost as well with her left. She constantly has to worry about others discovering her secret.
As far as I can tell on the internet, Tamora Pierce's Song of the Lioness books were a beloved fantasy series that shaped the adolescent years of many a young reader of both sexes. I never read them as a teenager, but can absolutely see why they are so adored. Alanna: The First Adventure has 524 five star reviews on Amazon, many from readers who discovered the books at a young age, and barely a negative review to be found. I wish I'd read the book when I was younger, but am very glad to have discovered the series now.