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The Cracked Throne by Joshua Palmatier (2006)
References to this work on external resources.
Wikipedia in English
Amazon.com Product Description (ISBN 0756404479, Mass Market Paperback)
Ruled by the Mistress of the Skewed Throne, the city of Amenkor has weathered attacks by invaders and the plague of madness brought by the mysterious White Fire. On the brink of civil war and mass starvation, the city turns to an inexperienced new Mistress, an orphan who has risen from the slums to claim the power of the Skewed Throne. Can Varis harness her survival skills and save Amenkor from ultimate destruction?
(retrieved from Amazon Thu, 12 Mar 2015 18:20:16 -0400)
After Varis was transformed unwillingly from gutterscum to assassin, she was made new Mistress of the Skewed Throne at the most perilous point in Amenkor's history. She and Eryn, the former Mistress, are haunted by a vision they shared in the madness after the White Fire, of an unknown enemy invading and destroying their city
(summary from another edition)