The Rise and Fall of Mount Majestic by Jennifer Trafton
Ten-year-old Persimmony Smudge leads (much to her chagrin) a very dull life on the Island at the Center of Everything . . . until the night she overhears a life-changing secret. It seems that Mount Majestic, the rising and falling mountain in the center of the island, is not a mountain at all--it's the belly of a sleeping giant, moving as the giant breathes. Now Persimmony and her new friend Worvil the Worrier have to convince all the island's other quarreling inhabitants--including the silly Rumblebumps, the impeccably mannered Leafeaters, and the stubborn young king--that a giant is sleeping in their midst, and must not be woken.
Dog Gone note: This book is in excellent condition.