Author: Diana Wynne Jones
Publisher: Collins, 1976
Power of Three is a children's fantasy book. It's written in her more serious style (it's comparable to Dogsbody and The Dalemark Quartet). The second half of the book starts moving towards her later, more playful style, but not anything like the style of Howl's Moving Castle or Deep Secret.
The Story (very minor spoilers)
Orban and his younger sister Adara are walking through the moor one day when they come upon a Dorig prince. Orban picks a fight with the Dorig, demanding its golden collar, but the Dorig naturally won't give it to him. Things escalate, and Orban ends up killing the Dorig, but not before it has put a powerful dying curse on the collar.
The story takes place a generation later, when Adaras children start coming into their powers (few people have powers, but Adaras children do -- and quite powerful ones at that). They discover that the collar's curse is threatening to destroy the moor, and everyone is too busy fighting the Dorig to do anything about it.
As they try to save the moor, they also discover that the Dorig aren't necessarily the enemy, and they also discover that the feared giants are also potential allies. But even with the three peoples working together, they may not be able to save the moor....
Like in other Jones books, this isn't so much as a quest as just normal people stumbling into strange circumstances and solving them as best as they can, with mistakes and confusions along the way. Definitely written in the style that makes Diana Wynne Jones so readable, but you may have to read through the first hundred pages before it really starts sounding like the classic Jones we know and love.