Agree with Diana Wynne Jones:
The Chrestomanci series:
Charmed Life
Witch Week
The Magicians Of Caprona
The Lives of Christoper Chant
Conrad's Fate
The Pinhoe Egg
Also
The Ogre Downstairs - a very unusual chemistry set causes amazing things to happen to the kids in the book - I especially loved the bit where the toffee bars come to life!
Howl's Moving Castle
Castle In The Air
House Of Many Ways
are all set in a world with 'fairy-tale' type devices - seven league boots,magic carpets, genies etc, but done in a very original and funny way.
Archer's Goon is about Howard who discovers their town is being run by 'seven megalomaniac wizards' - but they're nothing like the long beard and pointed hat wizards you would normally think of... very funny in places, gripping and a massive twist in it that I didn't see coming when I first read it.
The Power Of Three is a more serious adventure story, with more serious magic.
Also
The Spellcoats
Drowned Ammett
Cart And Cwidder
The Crown Of Dalemark
all set in another world
all set in another world, they have a more serious air about them too.