The Spellmaster's Book - Legends of Mytherios, #1

Hidden deep within the sewers of our world roosts a vile creature known only as The Dark Goblin. He holds in his possession The Spellmaster's Book, a text with the power to make him practically immortal. He fears no one, and turns all those who dare cross him into brainless goblins who obey his every command. Siblings Andy and Josie have heard rumors of The Dark Goblin and begin to fear he may be behind the mysterious disappearance of their father.
Together, they embark on a quest to reclaim the legendary Spellmaster's Book and free all of those who have been cursed by the malevolent goblin. As darkness closes in on Andy and Josie from all sides, they discover the greatest weapon they have to wield has been inside them all along.