The Isis Trilogy — Monica Hughes

The Keeper of the Isis Light is the first book in Monica Hughes' acclaimed trilogy, and it is the story of Olwen Pendennis, the Keeper of the Isis Light. She and her companion, Guardian, have lived alone on the remote planet Isis since the death of her parents. Now, colonists from an overcrowded and polluted Earth have landed and are settling in the valleys. Olwen fears these new inhabitants may ruin her world forever, and it seems she may
be right.

Years later, in Hughes' sequel, The Guardian of Isis, the planet remains pristine and remote, even after the arrival of settlers from Earth. Over time, the settlers have abandoned the technological knowledge of their ancestors and replaced it with myth. But one young man seeks the truth, and he soon gets the chance to find it.

The final chapter in the Trilogy, The Isis Pedlar, brings the devious Michael Joseph Flynn and his daughter, Moira, to Isis. Michael's magic firestone, his strangely delicious ambrosia, and his mysterious Forever Machine are beguiling to the humble members of the Isis community. Only Moira can expose this smooth-talking pedlar for the charlatan that he is. But will she be able to do it in time to save the simple agricultural community?