There are 3 books in the Sea of Trolls Trilogy series. The newest book is The Islands of the Blessed which came out in 2009.

1
The Sea of Trolls

Book cover of The Sea of Trolls
Jack is an apprentice bard and just beginning to learn the secrets of his mysterious master, when he and his little sister are captured by Viking chief, Olaf One-Brow, and taken to the court of Ivar the Boneless. Ivar is married to a half-troll named Frith, an evil and unpredictable…
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2
The Land of the Silver Apples

Book cover of The Land of the Silver Apples
In trying to save his sister Lucy from being thrown down a well, Jack has managed to cause an earthquake which demolishes a monastery. It appears that sometimes, the magic just doesn't quite work out... but then he is, after all, only a bard-in-training!

So when Lucy is unceremoniously carried…

3
The Islands of the Blessed

Book cover of The Islands of the Blessed
In the final adventure of Nancy Farmer's acclaimed trilogy a malevolent spirit of a vengeful mermaid is wreaking havoc on Jack's village and it's up to him, the old Bard and Thorgril to confront and vanquish the restless draugr. But the task will not be easy and the three find…