High Wizardry

When Dairine, younger sister of wizard Nita Callahan, finds Nita's copy of the wizard's manual, she takes the Oath herself before anyone can stop her, her own manual taking the form of a computer she names Spot. Carelessly powerful, she jumps across worlds, from Mars to a stranger crossing point between realities, until a hasty escape strands her on a planet made entirely of silicon, where she wakes a vast buried machine intelligence that starts spinning off living creatures of its own. Nita and Kit set off after her before the Lone Power can turn her discovery to its own ends. Third novel in Diane Duane's Young Wizards series.