You can get Diana Wynne Jones entire Chrestomanci series for £15 and it's a delight. Charmed Life is a my favourite.
She wrote a lot about the cruelty she experienced as a child, often at the hands of adults. So her books often feature kids having to navigate the problems the adults have left them.
Can I recommend:
Susan Cooper's The Dark is Rising
Diana Wynne Jones' Howl's Moving Castle (also, everything)
Diane Duane's So You Want to Be a Wizard
Terry Pratchett's The Colour of Magic (also, everything)
Ursula le Guin's The Wizard of Earthsea (also, everything)