Her debut novel, The Horizontal Man, was based in part on her experiences while attending Smith College. A philandering professor is murdered in his home and the suspects (students and staff) are either mad, lovesick, or rivals of the victim. In 1947 the Mystery Writers of America awarded it the Edgar for Best First Novel. It was one of the earliest examples of the psychological suspense thriller. Eustis once joked that she had written the novel because "she knew so many people in college she would like to murder".
Her 1954 novel, The Fool Killer, was turned into a movie starring Anthony Hopkins and Eddie Albert. And this year, Sarah Weinman included The Horizontal Man in her acclaimed Library of America collection, Women Crime Writers of the 1940s and 50s.