http://www.thebookseller.com/control/?p=21
http://www.bookfind-online.com/
As drchasuble says, The Bookseller lists top sellers each week, but as far as know (and I see it most weeks), they don't have any kind of information service for the rest.
Bookfind Online is operated by Nielsen Bookdata, the ISBN people, but you have to pay to use their service and even then, I don't think they list sales figures.
Amazon would list the book's place in its own rankings, which is a useful guide given how many people use Amazon, but for the real information you're going to have to contact the publisher, I think.