The most current statistics available on book publishing come from from R.R. Bowker, publisher of Books In Print. Bowker reported that, in 2003, 175,000 titles were published in the U.S., a new record.
The pertinent line from the release is this:
51% of all new titles published in 2003 by the three largest print-on-demand publishers were fiction, poetry or drama. These categories accounted for 57% of all titles published since 1998 by the same POD houses.
As with most statistics, caution should be exercised in reading these. Bowker's definition of "published books" consists of books that have been assigned ISBNs. For reasons open to debate, Bowker is the only authorized source of ISBNs in the U.S.
Reporters frequently ask how many titles are "self-published" annually. But the distinction, as horrifying as it may seem to those in the conventional publishing business, is really a fiction. There is no such thing as a self-published book, because there is no way, meaningfully, to distinguish a 'self-published' book from any other book.
You can distinguish a book with an ISBN from a book without -- Bowker's total of 175,000 titles omits the thousands of titles that are bought and sold that do not have ISBNs -- but you can't really distinguish an author publishing his own book from a friend publishing an author's book for him, or a small company publishing his book. Many authors set up publishing companies just to distribute their own books. And many small publishing companies distribute books by only a very small number of authors.
In book publishing statistics, generally the surveyer groups independent and self-publishers together. In the statistic cited above, Bowker dodges the question by distinguishing POD books from non-POD books rather than self-published books from published books. What they are really counting, of course, is POD books with ISBNs. By "the three largest print-on-demand publishers" they mean Bowker's three largest customers for ISBNs.
Lulu.com provides authors with a way to go around the Bowker monopoly. I would argue that Lulu.com is in reality the second largest POD publisher currently (with around 10,000 titles) and that by the end of 2004 it may be the largest.