Scribd is opening a new chapter by adding hundreds of books to its rapidly growing Web site for sharing documents. Random House and Simon & Schuster are among the long-established book publishers embracing Scribd as a new distribution and promotional channel. The partnerships being announced Wednesday will expand the eclectic mix of written material that has turned Scribd — pronounced “scribbed” as in scribbled — into one of the Internet’s hottest destinations just two years after it started.