This chart from Michael Hyatt represents the market share of the top 10 book publishers* in the US. Random House is the top publisher in US followed by Pearson.
[*] Publishers whose books are primarily sold through retail booksellers as opposed to, say, textbook publishers.
Pearson, which includes Penguin among other imprints, has remained at #2. Hachette has moved up from #5 to #3, no doubt as a result of the success of the Twilight series. As a result, HarperCollins and Simon & Schuster each moved down one spot. Thomas Nelson maintained its position at #7, but our market share dropped by 150 basis points. Tyndale fell off the list completely. Scholastic fell from #6 to #8, as a result of the sales decline in the Harry Potter series.