The Wizards of Wyrd World — Pamela F. Service

Josh Higgins and his sister Maggie have vowed not to use the alien gizmo that lets them visit other worlds. But when Josh's favorite writer comes to town, the temptation is too much. Along with famous author P.L. Cuthbertson, Josh and Maggie prepare to visit the land of Cuthbertson's books: Wyrd World.

As soon as the crew arrives, some of Wyrd World's thuggish locals mistake P.L. for a wizard--and that's not a good thing. Josh and Maggie begin a rescue mission, sneaking through fortresses and dungeons. But to challenge the planet's evil rulers, they'll have to unite a group of rebels and outcasts who would just as soon fight each other...