From the back cover:
"David and Ellen came to the lonely beachside cottage in hopes of rekindling their troubled marriage. Yet they are not alone on their second honeymoon. Marianna, a beautiful and enigmatic stranger, comes to visit when Ellen is away. But who is Marianna, and where is she from?
"Even as he succumbs to her seductive charms, David realizes that Marianna is far more than a threat to his marriage, for her secrets lie deep in the past and beyond the grave. And her unholy desires endanger the lives and souls of everyone she touches."
This is one of Matheson's lesser novels. Even Matheson's sharp prose fails to elevate this predictable ghost story above its clichés.
If there's a bright spot in this novel (aside from Matheson's crisp writing), it's the philosophical musings of his protagonist, David, who ponders the shapes marriage takes over time. These sections are wonderful, touching, and wise, and made up for my boredom during the other parts. (Fortunately, Earthbound is relatively short, less than three hundred pages.)
Not entirely bad, I'd hesitate to recommend this one. Better to read Matheson's earlier, short fiction if you want a Matheson fix.