The Tangle Box by Terry Brooks
Horris Kew, conjuror and confidence-trickster, had returned to the Magic Kingdom of Landover, and a chorus of disapproval rang in Ben Holiday's ears. Both Questor Thews, the Court Wizard, and Abernathy (the Court Scribe who had magically been transformed into a dog) urged Ben to lock him up or send him away. Fair as ever, Ben decided to consider the problem rather than act immediately. Alas, this led to disaster, for Horris had not returned of his own volition, and been Holiday soon found himself in deadly danger to which only his lady, Willow, held the key. But Willow had disappeared on a mysterious mission of her own ...