The Secret of Ka by Christopher Pike
From the electronic description: One minute Sara's bored on vacation in Istanbul. The next, she's unearthed a flying carpet that cleverly drags her to the mysterious Island of the Djinn - or genies. By her side is Amesh, a cute guy she has a crush on but doesn't yet trust. When Amesh learns the secret of invoking djinn, he loses control. He swears he'll call upon only one djinn and make one wish. The plan sounds safe enough. But neither Sara nor Amesh are any match for the monster that swells before the. It hypnotizes Amesh, compelling him to steal Sara's flying carpet and leave her stranded on the island.
Discovering the carpet has sparked a new path for Sara, one that will lead her to battle creatures even deadlier than djinn. In this fight, Sara can save mankind, herself or the boy she cares for. Who will she be forced to sacrifice?