Jack Sparrow: The Siren Song

Pirates of the Caribbean: Jack Sparrow: The Siren Song is the second in the series of young reader books written by Rob Kidd dealing with the early life of Jack Sparrow. It was released on June 1, 2006.

Publisher's Summary
Before the Black Pearl, there was a teenage stowaway named Jack Sparrow....

Captain Jack Sparrow's ship, the Barnacle, and its up-till-now unshakable crew have suddenly fallen under a sinister spell. While continuing on their quest for the storied Sword of Cortés, the crew suddenly finds itself entranced by an ethereal song. As a result, each of them attempts to take the Barnacle in a different direction—and away from the Sword’s supposed location. Only Jack seems unaffected by the strange song, but can he both subdue his crew and defeat the mysterious force behind the dark spell?

Summary
Then, after many days at sea, the crew of the Barnacle begin to act very strange after hearing a phantom-esque song floating over the waves. Each crew member wants to take the Barnacle in a different direction and away from the Sword's location. Jack seems unaffected by the Song, and wonders what is afoot. He is then taken deep below the ocean, after strange creatures start attacking the Barnacle and confronted with antagonist mermaids in an undersea mermaid kingdom. Jack says that he and his crew are after the Sword of Cortés. The mermaids then make a deal with Jack that when the crew eventually gets the Sword, they would have to return to the mermaid kingdom and give them the thing Jack values most. (Although they state that it is not physical treasure.) He also gives them the stone eye - which he does without thinking. Jack is then set free and his crew goes back to normal. It is a little while later, Jack realizes to his horror that the thing he values most is his freedom. (He has essentially sold himself to the Sirens.)