These are the books by Diana Wynne Jones that are reviewed on this site:
David meant to say the curse, but he never expected it to work! Now he's got a rather strange kind of friend who doesn't live by the same rules as everyone else . . .
In a divided kingdom Clennen the Singer travels from village to village carrying news. But when he agrees to take a passenger up to the north things begin to go wrong.
High Treason at Court. A plot to bespell the King. The Merlin is implicated. Who can Roddy and Grundo turn to for help? A magician from another world, of course.
Chrestomanci the great enchanter takes orphans Gwendolen and Cat under his wing, but living with a great enchanter isn't as enchanting for Gwendolen as she had supposed.
Gair's father is a hero and a chief, and his brother and sister have astonishing powers, but Gair is just Gair - ordinary. But ordinary Gair finds that he is the one who might just be able to save his people from the warring Dorig, and the Giants.