This question is not within our guidelines - we usually limit ourselves to questions from activists or those interested in social issues, or independent journalists. Please consider some of the resources we mention on our "ask a question" page. But here's a couple of ideas.

The most direct source for this information would be Nielson, which has a book tracking service. They would want you to pay for the information. You can find some other ideas on this Google Answers thread from 2002.

Author Cheryl Swanson discusses why this question is a challenge in this blog entry.