Answer: Emma (E. D. E. N.) Southworth. She was born in Washington in 1819, back when the city was still little more than a wilderness with four towns in it: Georgetown, Lafayette Square, Capitol Hill, and the Navy Yard. A teacher, she took up writing in 1844 to earn extra money, after her husband left her and their children. Her first novel, published in 1849, sold 200,000 copies; she would write over sixty more. Many of her potboilers were first serialized in large-circulation magazines, a common practice well into the 20th Century.
According to this "multi-media archive" of Stephen Railton, Professor of English at U-Virginia: "In our time she is best known by the novel The Hidden Hand, originally published in The [New York] Ledger in 1859, and reprinted there twice before it was finally issued as a book in 1888."
Southworth's an example of those commercially-minded artists who were wildly successful in their own time but are now all but forgotten. Today's equivalent is probably something like the somewhat more anonymous writers of the teleplays for popular TV dramas, soaps, and telenovelas.
Incidentally, Southworth, who died in 1899 and is buried at Oak Hill Cemetery, lived for years in a Georgetown house (now demolished) that stood in roughly the same spot as the fictional one in The Exorcist (1971).