(Note that these statistics are for print books only, they do not take ebooks into account.)
"The United States' book market grew to $10.71 billion in 2007 [...] R.W. Bowker figures show U.S. title output increased to 276,649 new titles and editions, up from 274,416 in 2006. When you add in the 134,773 "on demand" and short-run books to these numbers, there is a projected output of 411,422 book in the United States in 2007."
The report lists the following data for the 2007 U.S. consumer book market:
~ $1.375 billion in estimated romance fiction revenue (which gives it 12.8% of the total book market)
~ Romance fiction sold about twice as much as Mystery or Science Fiction/Fantasy, three times as much as classic literary fiction
~ about 8090 romance books released, a 25.9% increase over 2006
~ 318 romance titles by 168 authors hit the best seller lists (NYT, PW, USA Today)
~ The top five romance book publishers were Harlequin, Random House, Penguin Group, HarperCollins, and Kensington.