The Sapling Cage by Margaret Killjoy

In the gripping first novel in the Daughters of the Empty Throne trilogy, author Margaret Killjoy spins a tale of earth magic, power struggle, and self-invention in an own-voices story of trans witchcraft.
Lorel has always dreamed of becoming a witch: learning magic, fighting monsters, and exploring the world beyond the small town where she and her mother run the stables. Even though a strange plague is killing the trees in the Kingdom of Cekon and witches are being blamed for it, Lorel wants nothing more than to join them.
 
There’s only one problem: all witches are women, and she was born a boy.
When the coven comes to claim her best friend, Lorel disguises herself in a dress and joins in her friend’s place, leaving home and her old self behind. She soon discovers the dark powers threatening the kingdom: a magical blight scars the land, and the power-mad Duchess Helte is crushing everything between her and the crown. In spite of these dangers, Lorel makes friends and begins learning magic from the powerful witches in her coven. However, she fears that her new friends and mentors will find out her secret and kick her out of the coven, or worse.