Windrose Chronicles, book 2: The Silicon Mage
Barbara Hambly1988

It was impossible, Joanna knew. When she betrayed her lover Antryg Windrose to the Council, he had been sentenced to death. Then she had believed his brain was possessed by the Dark Mage Suraklin, though now she knew that Suraklin had chosen Gary Fairchild. But guilty or innocent, Antryg was separated from her by the awesome Void between the worlds, far from any hope of rescue.

Nevertheless, she had to save him. Suraklin was planning to gain immortality by placing his mind in a computer that would get its power by draining the life-force from all on both worlds, dooming everyone to eternal misery and hopelessness. And only Antryg was strong enough a wizard to challenge the Dark Mage.

Once again, Joanna dared the fearsome tunnel through the Void, praying desperately that Antryg still lived and that she could find help to free him. If not...But she refused to think of that.