Authors uploading to Amazon and looking into ways to promote their title/s will eventually hear about the Publisher Rocket service. Publisher Rocket is a book marketing tool focusing solely on Amazon (which is the dominant online bookseller for both print and ebooks).
In the words of an author reviewer:
Publisher Rocket is essentially a research tool. It helps self-published authors understand what readers actually want, and how to connect with them with your books. But it's not just for self-published or independent authors—traditional publishers use it too.
Here are some things you can discover in Publisher Rocket:
What phrases Amazon buyers are actually searching for.
The psychology of how readers choose to buy books.
How much money readers are spending on certain niches and topics.
How much money specific books are making per month (for example, books that might be competing with yours).
Reddit has an interesting thread discussing the pros and cons of the service. Reviews on Trust Pilot are quite positive. To be used in conjunction with Dave Chesson’s blog (Chesson is the founder of Publisher Rocket)