In 1991, a book editor at Random House pulled from the heaps of unsolicited manuscripts a novel about a murder that roils a Baltimore suburb. Written by a first-time author and mother named Mary Cahill, "Carpool" was published to fanfare. Ms. Cahill was interviewed on the "Today" show. "Carpool" was a best seller.

That was the last time Random House, the largest publisher in the U.S., remembers publishing anything found in a slush pile. Today, Random House and most of its major counterparts refuse...