Unicorn Series by Tanith Lee, Mark Zug

It was big and beautiful and so black that it was like a hole in space, and it was completely impossible. Everybody knew there were no unicorns. Unicorns didn't belong in this world except in legends. But there it stood, radiating magical power, in the shattered wreck of the party.Nobody knew where it had come from, or what it wanted. Not even Jaive, the sorceress, could fathom the mystery of the fabled beast. But Tanaquil, Jaive's completely unmagical daughter, understood it at once... moreIt was big and beautiful and so black that it was like a hole in space, and it was completely impossible. Everybody knew there were no unicorns. Unicorns didn't belong in this world except in legends. But there it stood, radiating magical power, in the shattered wreck of the party.Nobody knew where it had come from, or what it wanted. Not even Jaive, the sorceress, could fathom the mystery of the fabled beast. But Tanaquil, Jaive's completely unmagical daughter, understood it at once. She knew why the unicorn was there: It had come for her. It needed her. Yet she was the girl with no talent for magic. She could only fiddle with broken bits of machinery and make them work again. What could she do for a unicorn? less

Tanaquil, a young mender, and her familiar come face to face with her half-sister, Lizra, who forces Tanaquil to make a perilous choice between siding with Lizra in her quest for conquest or risking her terrible anger.

The enchanting, magical sequel to The Black Unicorn and The Gold Unicorn!After several years traveling, Tanaquil — a sorceress like her mother with the ability to mend — returns home only to discover that her true love has been betrothed to her sister, Empress Lizra. Broken-hearted and jealous of her sister’s happiness, Tanaquil is lured by a red unicorn into a mirror world where she encounters Tanakil, a diabolical version of herself. She discovers also several new powers... moreThe enchanting, magical sequel to The Black Unicorn and The Gold Unicorn!After several years traveling, Tanaquil — a sorceress like her mother with the ability to mend — returns home only to discover that her true love has been betrothed to her sister, Empress Lizra. Broken-hearted and jealous of her sister’s happiness, Tanaquil is lured by a red unicorn into a mirror world where she encounters Tanakil, a diabolical version of herself. She discovers also several new powers. Powers she will need to foil Tanakil’s sinister plot of revenge. less