This site offers invaluable information about monthly comic book sales, current and past, starting from 1995. Want to know which was the top selling comic book in july 2003? Easy, Batman issue 617 with approximately 146,000 copies.
In the direct market there is basically only one distribution game and if you want to play, Diamond is your only real option. They publish the previews catalog, which enables fans to pre-order comic books from shops, which in turn lets publishers calculate the size of their printing run.