Elemental Masters, book 3: The Gates of Sleep
Mercedes Lackey2002

For seventeen years, Marina Roeswood had lived in the care of close friends of her wealthy, aristocratic parents. As the ward of bohemian artists in turn-of-the-century England, she had grown to be a free thinker in an environment of fertile creativity and cultural sophistication. But the real core of her education was far outside societal norms. For she and her foster parents were Elemental Masters of magic, and learning to control her growing powers was Marina's primary focus.

But though Marina's life seemed idyllic, her existence was riddled with mysteries. Why had she never seen her parents, or been to Oakhurst, her family's ancestral manor? And why hadn't her real parents trained her themselves?