There are 2 books in the Elana series. The newest book is The Far Side of Evil which came out in 2003.

1
Enchantress from the Stars

Book cover of Enchantress from the Stars
Rediscover this beloved Newbery Honor-winning classic, Featuring a brand-new cover and a foreword by Lois Lowry!

Elana, a member of an interstellar civilization on a mission to a medieval planet, becomes the key to a dangerous plan to turn back an invasion. How can she help the Andrecians, who still…
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2
The Far Side of Evil

The Far Side of Evil
Please note: Although this book has the same heroine as the author's Newbery Honor book Enchantress from the Stars, it is completely independent and is intended for older readers, high school age and adults.  The 2003 reissue is an updated edition and should be read in preference to earlier editions.…