Conrad Mason 
The Goblin »s Gift 
Tales of Fayt, Book 2

Joseph Grubb is the newest member of the Demon »s Watch. He and his fellow watchmen protect Port Fayt, where humans live in peace alongside trolls, elves and fairies. And now the town needs them more than ever, because the almighty League of the Light has sent an armada to wipe it off the map.
Fayt »s only hope is to persuade the magical merfolk to fight with them. But the merfolk won »t go to war. Not unless their princess is returned to them from the clutches of the most dangerous nine-year-old in the Ebony Ocean.
It »s up to Joseph and his friend Tabitha to rescue the mermaid princess . . . But a secret from Joseph »s past is about to change everything.