Reference sources:

450,000 English-language books published in the world during 2004, 375,000 in English-speaking countries:
http://www.bowker.com/press/bowker/2005_1012_bowker.htm

181,189 books published in US in 2004:
http://www.bookwire.com/bookwire/decadebookproduction.html

46,738 – US, 1990; 15,012-1960; 1900-6356
http://www.clir.org/PUBS/reports/pub103/appendix1.html

90% of books sell fewer than 1000 copies
http://www.csmonitor.com/2005/0331/p01s01-stin.html

2002 statistics:
http://www.bookwire.com/bookwire/BookProduction/DecadeBookProduction.html

Only 2% of books sold more than 5000 copies:
http://www.anvilpub.com/sr_oct_2003.htm