The Emperor's Mistress - Larenia's Shadow Trilogy, #1

Teenaged Stealth is an artful thief with an attitude. Hired to burgle an exiled mage's villa, she tangles with Derrius, an apprentice mage of revoltingly noble birth. Trapped by political upheaval and their own antipathy, Stealth and Derrius undertake a mission to avert civil war. The two must learn to work together and dovetail her thieving skills and his magic. Long ago, Larenia's Sword protected their nation from evil, but the blade has been lost for a hundred years.
Now, evil arises anew in the sultry form of the emperor's mistress, Illisandra Zayla. If Stealth and Derrius fail to find and recover the Sword, the sorceress will enthrall the emperor and seize the power her tormented soul lost long ago.