Mariana by Susanna Kearsley
The first time Julia Beckett saw Greywethers she was only five, but she knew that it was her house. And now that she's at last become its owner, she suspects that she was drawn there for a reason. As if Greywethers were a portal between worlds, she finds herself transported into seventeenth-century England, becoming Mariana, a young woman struggling against danger and treachery, and battling a forbidden love. Each time Julia travels back, she becomes more enthralled with the past...until she realizes Mariana's life is threatening to eclipse her own, and she must find a way to lay the past to rest or lose the chance for happiness in her own time.