I am looking for the total number of baseball-related books sold
in 2003 (or whatever is the most recent year for which data are
available), and the gross revenue from those sales. I also need to
cite the source for those figures. A perfect answer would look
something like this: "According to the American Booksellers
Association, in 2003 2.5 million baseball books were sold for a total
of $45 million." Thanks for your help.

This article doesn't precisely answer your question, but I think
you'll find it interesting:

http://www.baseballhalloffame.org/membership/memories_dreams/2003/fall/bestsellers.htm