There are 4 books in the Quarters series. The newest book is The Quartered Sea which came out in 1999.

1
Sing the Four Quarters

Book cover of Sing the Four Quarters
The Bards of Shkoder hold the country together. They, and the elemental spirits they Sing – earth, air, fire, and water - bring the news of the sea to the mountains, news of the mountains to the plains. They give their people, from peasant to king, a song in common.…
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2
Fifth Quarter

Fifth Quarter
Planning to assassinate the evil Gyhard, siblings Bannon and Vree of the Havalkeen army face a terrible choice when Gyhard steals Bannon's body and forces the pair to coexist in Vree's body

3
No Quarter

Book cover of No Quarter
Bannon and Vree, a brother-and-sister assassin team, must share Vree's body when Bannon's is stolen from him and begin a desperate quest into a neighboring kingdom in order to regain Bannon's body from Gyhard. Original.

4
The Quartered Sea

The Quartered Sea
When an expedition into the uncharted waters to find the homeland of the legendary Dark Sailor ends in disaster, Benedikt, a young bard, finds himself stranded in a mysterious unknown world, the pawn in a deadly game of politics and religion that destroy a kingdom's precarious balance of power. Original.