Where can I get data on trending textbook sales?

I'm looking for a service I can query to get the most popular math textbooks every semester or so.

Does Amazon or Ebay have some sort of API I can query? I haven't found anything and I'm assuming this kind of data is valuable, so it's kept private. If so, what suggestions do you guys have for a one man show to get data on popular math textbooks for high school and college students?

Thanks!