My theory is that we're just looking at the list of books most frequently assigned by english teachers. Excluding children books, most of these can be found in the new york state education department's literature suggestions: http://www.p12.nysed.gov/guides/ela/part1b.pdf.

The exceptions were "How to Win Friends and Influence People" (rank 8) and "Harry Potter" (rank 9).

I am actually pretty shocked by this. I didn't expect a self-help book (15 million copies sold) to beat harry potter (120 million copies sold, 500 million if you include the entire series, making it the best-selling series in history). I guess people who love the franchise would rather own the book?