Elana Series by Sylvia Engdahl, Lois Lowry

Elana is a member of a supremely advanced interstellar civilization, on a mission to the medieval planet Andrecia. To her shock, she becomes the key to a dangerous plan to turn back an invasion by an aggressive, space-faring "Youngling" species... moreElana is a member of a supremely advanced interstellar civilization, on a mission to the medieval planet Andrecia. To her shock, she becomes the key to a dangerous plan to turn back an invasion by an aggressive, space-faring "Youngling" species. How can she possibly help the Andrecians, who still believe in magic and superstition, without revealing her alien powers?Apprentice Medical Officer Jarel knows that the Imperial Exploration Corps doesn't consider the Andrecians to be human, and he has seen the atrocious treatment the natives get from his people. How can Jarel make a difference, when he alone regrets the destruction his people bring?Georyn, son of an Andrecian woodcutter, knows only that there is a dragon on the other side of the enchanted forest, and he is prepared to do whatever it takes to defeat it. To him, Elana is the Enchantress from the Stars who has come to test him, to prove his is worthy.... less

On completion of her training as an agent of the interstellar federation's Anthropological Service, Elana is sent to a world whose people may soon destroy their civilization. Since not enough is understood about the situation to justify any interference with their evolution, the Service has no power to act; its agents must go as helpless observers, posing as natives, in the hope of gaining knowledge that may help to save other worlds... moreOn completion of her training as an agent of the interstellar federation's Anthropological Service, Elana is sent to a world whose people may soon destroy their civilization. Since not enough is understood about the situation to justify any interference with their evolution, the Service has no power to act; its agents must go as helpless observers, posing as natives, in the hope of gaining knowledge that may help to save other worlds. This passive role proves intolerable to the young, inexperienced agent assigned to the same city as Elana, a city under totalitarian rule. After falling in love with a local girl who has become Elana's closest friend, he identifies too completely with the natives and unwittingly endangers the entire world by a well-meant but ill-advised attempt to intervene. Forced to assume responsibility for undoing the damage, Elana finds that only she—at great cost—can prevent an immediate war of annihilation.Although this novel has the same heroine as the author's Newbery Honor book Enchantress from the Stars, it is not a sequel but a completely separate—and very different—story not intended for readers below high school age, and is also enjoyed by adults. The two books are in no way dependent on each other and can be read in either order. less