U.S. Big Book Publishers and Who Owns Them

It's a well-known fact that most of the major publishing houses have consolidated and merged and rearranged themselves many times over. This has affected magazines as well as book publishers, and many of the holding companies aren't in the U.S.
Simon & Schuster (S&S), founded in New York City in 1924, is currently owned by CBS. Still headquartered in NYC, it is a bastion of fiction and nonfiction, producing more than 1000 titles a year from 35 different imprints, including Pocket Books, Scribner, Atria, Fireside, Touchstone, and Atheneum Books for Young Readers.
Baen Books is a more recent stalwart of American publishing. It was founded in 1983 when a huge reorganization of Simon & Schuster was underway. S&S approached Jim Baen with an offer for him to head up the S&S science fiction line (Pocket Books division). Baen, however, had different plans. He obtained financial backing from some friends and proposed to start a new company named Baen Books. The deal was done and, at the beginning, Simon & Schuster handled the distribution. Nonetheless, many of the book publishers we might assume are "American" no longer hold that distinction.

Examples:
• Houghton Mifflin Company, founded in Boston in 1832, is now Houghton-Mifflin-Harcourt (HMH), with the company having purchased Harcourt (formerly Harcourt Brace Jovanovich) in 2007. Several mergers and buyouts have ensued and HMH is now owned by Education Media and Publishing Group (EMPG), an international holding company registered in the Cayman Island. HMH is a leader in the educational publications marketplace. NOTE: Although primaries in EMPG are from Ireland, one of the major investors is Guggenheim Partners, a U.S. investment corporation.
• Alfred A Knopf, Inc founded in 1915, was purchased by Random House in 1960. Random House also bought Doubleday, and now there's a KnopfDoubleday company under the RH umbrella. It's a publishing consortium of its own, with a half dozen or so imprints. Random House, Inc. has been owned since 1998 by the German mega-media company, Bertelsmann.
• The Free Press was founded in 1947, became an imprint of Simon and Schuster, was sold in 1960 and merged into the Macmillan Publishing Company. Macmillian, founded in London, opened its first U.S. offices in 1869. It is now part of the large German holding company, Georg von Holtzbrinck.

The publisher umbrellas have a wide span, with most of the imprints belonging to three or four houses. Random House, S&S, Harper-Collins, and Penguin Group (USA) produce the majority of books we see on commercial bookshelves across the country. Former companies have become imprints: Farrar Straus, Henry Holt, Little Brown; and some have vanished (Fawcett, Carol Graf, Arbor).
But there are plenty of small and Indie publishers. Check out the Wikipedia pages to see the more than 500 companies that are competing with the big guys.