The Glass Flame — Phyllis A. Whitney

"If anything happens to me down here, don't let it pass as an accident. You owe me that, Karen..." David Hallam had written those words in his last letter to his wife from a small village in the Tennessee Smoky Mountains. Ten days later he was dead. Now Karen Hallam had to find out why.
Her search would take her deep into the misty, haunted mountains where her husband was born, and where he spent the last few weeks of his life. It would lead her into a tangled web of disputed fortune, family jealousy, conspiracy, adultery and murder. And it would bring her face to face again, with Trevor Andrews, David's half brother-the first man she had ever loved.
But Trevor was married now, distant, unreachable-and before long Karen would learn that Trevor Andrews had his own good reasons for wanting David dead. And in that moment she would also know that her passion for the truth might drive her to betray the deepest instincts of her heart.