Heh... authors have been trying to find out this same information since publishing began! Even semi-annual royalty statements don't tell an author how many books are SOLD. The publisher will only tell you how many have been
shipped, and
returned, but what might still be on the shelf in any particular store is unknown. BookScan has bits of the information. Mind you, they have the MOST bits of anyone, but lack significant secondary markets like Wal-Mart, Target, mom-and-pop groceries and independent bookstores (unless they happen to use a major distributor, like General Independents, to get their books.) And BookScan is
EXPENSIVE! Each individual list (like Romance, or Science Fiction) costs thousands of dollars per year. I think I heard once that the FULL list is in the quarter million per year range.
It's frustrating, but there you go...