Diadem: Shadowsong Series by Jo Clayton

This is the first in The Shadowsong Trilogy, the story of Shadith, formerly a spirit residing in the Diadem, a magical relic from an alien empire. She is embodied with the gift of empathy and a magic based on music, which enables her to act as an arbiter for interplanetary diplomatic corps. Now she must make peace between two warring races.

Shadith once existed as a spirit entrapped in the Diadem, an enigmatic alien artifact. Now free, Shadith finds herself with the ability to transfer her consciousness into another--becoming the perfect spy. Hired to recover a valuable technological device, she stumbles across one of the greatest crimes in the history of the known galaxy.

As an empath, Shadith is the perfect employee of Excavations Limited, a high-tech private detective agency. But when assigned to locate and return a creature who has been abducted by a smuggler, Shadith cannot in good conscience fulfill her mission. Not when it means returning the creature to a people who mean to destroy it. Conclusion of the Shadowsong trilogy.