Synopsis from Goodreads

Half Bad by Sally Green is a breathtaking debut novel about one boy's struggle for survival in a hidden society of witches. You can't read, can't write, but you heal fast, even for a witch. You get sick if you stay indoors after dark. You hate White Witches but love Annalise, who is one. You've been kept in a cage since you were fourteen. All you've got to do is escape and find Mercury, the Black Witch who eats boys. And do that before your seventeenth birthday. Easy.