Merlin Codex. Book One, Celtika

In the centuries before he will meet Arthur, Merlin is both immortal and young. He journeys across this world - and worlds unknown. He becomes a friend of the Greek hero Jason, who seeks the Golden Fleece, and later of his wife, the enchantress Medea. When hate and treachery drive them apart, Merlin wanders until a chance meeting leads him to find and raise the hulk of the Argo, which contains the spirit of Jason. Thus a great quest begins, which ends in unlooked-for ways and causes Merlin to travel for the first time to the land which will become England.