« The Silicon Mage » Barbara Hambly

There was a time when Joanna Sheraton knew nothing of the Void. She was an ordinary computer programmer, toiling in a cubicle in air–conditioned Southern California comfort, unaware that sinister forces had penetrated her universe.

But from across the interdimensional divide, an evil mage had put in motion a scheme for eternal life, by transferring himself into a computer that feeds on Earth’s life force.

Called upon to help by the wizard Antryg, Joanna could do nothing more than delay. At the end of her first sojourn across the Void, Antryg was imprisoned and their task seemed hopeless. Now she must depart from Earth once more, to rescue Antryg and save humanity. She is friendless, and the dark mage’s forces hound her every step. But a good hacker is not easily deterred.