Windrose Chronicles, book 1: The Silent Tower
Barbara Hambly1987

Antryg Windrose, dog wizard student of the Dark Mage, has been imprisoned in the Silent Tower for seven long, lonely years.
But have his powers been limited by the spell-bound tower walls, or is the life-sapping Void and the appearrance of appalling abominations throughout the countryside somehow connected to him ?
And is he linked to the strange occurences noticed by young computer programmer Joanna Sheraton at the San Serano Aerospace Complex ? Joanna has the feeling that someone is following her, but who, and why ? At a party thrown by her boyfriend Gary, she is soon to find out : whirled through the Void into an unfamiliar world, accompanied by a mad wizard and a beautiful wordsman, she is about to discover depths within herself she had never dreamed of, and horrors worse than any nightmare...