Authors want to know how many of their books sell. If you publish a first book in a proposed series, for example, the sales numbers influence whether you receive a contract for additional books with the publisher. If you self-publish, tracking those numbers helps you decide whether the book warrants additional marketing or a follow-up. There are comparatively few ways to get accurate sales numbers.
Third-party sources theoretically track sales. In reality, however, third-party sources typically track information such as sales rank, which corresponds to the popularity of your book, rather than the number of books sold. In some cases, they track the number of books shipped from warehouses to retailers, but these numbers often prove higher than actual sales due to returns. The major source of third-party sales data is Nielsen BookScan but, as reported in Forbes, BookScan numbers often prove inaccurate.
Direct From the Publisher
For the traditionally published author, the publisher represents the only meaningful source of sales numbers for your book. The publisher’s internal numbers ultimately determine everything from your advances and royalty payments to your continued relationship with the publisher.
If you self-publish, you get an advantage over traditionally published authors in terms of tracking sales numbers. Most of the major self-publishing outlets for both print books and e-books provide you with regularly updated sales information, often broken down into daily and monthly sales. .