There’s no single source that will give you a comprehensive answer to this question. Neilsen BookScan provides data about sales to the industry and individuals but you have to subscribe to the service. Even then, the data you will receive does not accurately answer how many books are being sold. This is because the system relies on point-of-sale data from major chains and does not take in to account those sold through independents or by other avenues. There are websites which try to predict sales figures based on the sales rankings on Amazon but these are only theories and are largely untested. The only certain way of finding out the information is to contact the publisher of the individual book. This too must be treated with caution as even if the publisher co-operates with your request, and whether they do will be based on the reason you are trying to find out the information, as there is a difference between how many copies of the book has been published and how many have been sold. The publisher may only be able to give you the number that have been published which will be a greater amount than those sold as many will still be sitting in storage depots or on shelves in book stores.

The girl with the dragon tattoo

There is no particular way to find out how many copies have been sold. The only thing you could know is that how many copies have been published by the company. Looks at the book and find out, how many books have been published by the printing company.

I've only found one source that claims to provide up-to-date sales figures for individual titles, and that is Nielsen BookData. You can register for a free 30 day trial.