Lily Quench and the Lighthouse of Skellig Mor — Natalie Jane Prior

At the end of the earth is Skellig Lir, a dreamy enchanted island that is inhabited by mysterious people with strange powers. To get there, Lily Quench and Queen Dragon must brave magic, storms, and seas swarming with deadly sea dragons -- and Ariane, the rebellious keeper of the lonely lighthouse of Skellig Mor.
 
In the depths of an undersea cavern, Lily struggles to communicate with the sea dragons and escape the skeleton-filled tunnels beneath the lighthouse. Only then can she complete her quest: to enter the fantastical library that has existed since the beginning of the world.