The Copper Crown (The Tales of Aeron #1)

WHEN EARTH MEETS KELTIA WILL STAR EMPIRES FALL?
When lore became legend on ancient Earth and the powers of magic waned, the Kelts and their allies fled the planet for the freedom of distant star realms.

But the stars were home to dangerous foes, and millenia later, the worlds of Keltia still maintained uneasy truce with two enemy empires -the Imperium and the Phalanx. Then, at the start of the reign of Aeron, mistress of high magic and queen of all the Kelts, an Earthship made contact with her long-fled children. And while Earth and Keltia reached out to form alliance, the star fleets of the enemy mobilized for final, devastating war....