Sands and Tombs (Dragon Thief Book 4) - Dragon Thief, #4

Millie Lucas and her handsome dragon protector Benjamin Castle are about to find themselves in a very sandy situation. Their jaunt to the seaside port of Pernix is interrupted by the arrival of a stranger interested in their magic. Before they know it, they're whisked off to the island nation of Kalea where it's revealed that the king himself demands they give over a part of their spell-casting abilities for the sake of his kingdom.
Their unique system of protecting themselves is failing, and only the infusion of fresh magic will save it from collapsing. They're rescued from the theft of their magic by the vouching of Ben's cousin, a wealthy prince of the nation. Unfortunately, his cousin's position, too, has recently become shaky with the king. A horrible curse hangs over their house, brought upon by an unknown assailant, and their hiring of a hag, to prevent worse misfortune has alienated many allies, the royalty included. Now Millie and Ben must discover not only the source of the trouble for his cousin but what's causing the failing of the island's protective barrier.
Danger looms in the background as shadowy forces seek to stop them from finding the truths they seek, even if it means resorting to murder.