It's a simple 10 page stats guide by clickbank breaking down the data clickbank has gathered in terms of conversion rate by country, day of the week, price points and a few other goodies.
It's $1.99 and I suggest you get it. I'm not affiliated with it in ANY way.
It shows that the U.S. is #8 when it comes to conversion rate, this just shows that there is huge potential for high ROI campaigns outside of the U.S. where most marketers are.
Are you focused on the U.S. or are you running campaigns elsewhere?