Red Unicorn — Tanith Lee

The young wanderer Tanaquil can mend anything that is broken--except her own heart. With the engagement of Honj to her sister, Emperess Lizra, she returns home to her sorcerous mother--and her mother's new lover, the magician Worabex.

Caught up in their combined magic, Tanaquil and her mischievous familiar--a literal pet peeve--find themselves in a parallel world where she meets Tanakil, a mirror-image princess with murder on her mind.

With Red Unicorn, award-winning author Tanith Lee returns once again to her epic fantasy of magic, of alternate worlds both perfect and flawed, and of the enchanting unicorns that travel between them.