1988’s The Silicon Mage is the second volume in Barbara Hambly’s Windrose Chronicles.
In the belief that the malevolent mage Suraklin was possessing Antryg, Joanna Sheraton left the dread revenant and its host in the gentle care of the authorities in Suraklin’s native dimension. She returns home, across the dimensions, to California.
It is then that Joanna realizes that she has made a terrible mistake. Suraklin is not possessing Antryg; he’s ensconced himself in Joanna’s nasty boyfriend Gary. Antryg’s arrest did not neutralize Suraklin’s threat. It neutralized the only person able to stop Suraklin.