Reddit reviews Children of the Star

Children of the Star, by Sylvia Engdahl?

>Back in print! One-volume edition of the trilogy including the three novels This Star Shall Abide (known in the UK as Heritage of the Star), Beyond the Tomorrow Mountains, and The Doors of the Universe.
Noren knew that his world was not as it should be--it was wrong that only the Scholars, and their representatives the Technicians, could use metal tools and Machines. It was wrong that only they had access to the mysterious City, which he had always longed to enter. Above all, it was wrong for the Scholars to have sole power over the distribution of knowledge. The High Law imposed these restrictions and many others, though the Prophecy promised that someday knowledge and Machines would be available to everyone. Noren was a heretic. He defied the High Law and had no faith in the Prophecy's fulfillment. But the more he learned of the grim truth about his people's deprivations, the less possible it seemed that their world could ever be changed. It would take more drastic steps than anyone imagined to restore their rightful heritage.

Edit: Original cover can be seen here on the author's official page.