KDP Spy is a Chrome extension from Book Bolt, a research tool for authors who publish through Amazon Kindle Direct Publishing (KDP). On an Amazon search results page it shows estimated sales, revenue and ranking data for the books listed, which helps you judge whether a niche has demand and how crowded it is. It has about 50,000 users, but its Chrome Web Store rating is low, around 2.4 out of 5 in September 2026, so it is worth knowing its limits before relying on it.
What KDP Spy shows
- Best Sellers Rank for each book in the results.
- Estimated monthly sales and revenue, derived from that rank.
- Price, page count, review count and publication date.
- A summary of the whole page, to compare niches quickly.
How to use KDP Spy for niche research
- Install KDP Spy from the Chrome Web Store and sign in with a Book Bolt account.
- Search Amazon for your book idea, for example a type of journal or a non-fiction topic.
- Open the extension on the results page and look at the ranks and estimated sales of the top books.
- Repeat for related searches and compare: you want steady demand without only big publishers at the top.
What a good niche looks like
| Signal | Encouraging | Warning sign |
|---|---|---|
| Best Sellers Rank of top results | Several books selling steadily | Only one or two books selling at all |
| Review counts | Some sellers with few reviews | Every top book has thousands |
| Publishers | Independent authors in the top results | Only large publishing houses |
| Publication dates | Recent books selling | Only very old titles, suggesting fading interest |
How to read Best Sellers Rank
Almost every number the extension shows starts from Amazon’s Best Sellers Rank (BSR), so it helps to know how BSR behaves. A lower rank means more recent sales: a book ranked in the low thousands in the Kindle Store sells far more than one ranked in the hundreds of thousands. The rank reacts quickly, so a single good day can lift a book for a while and then it slides back. Amazon also shows category ranks, which are easier to top than the overall store rank. For niche research, look at the ranks of several books over a week or two rather than a single snapshot, and compare paperback and Kindle ranks separately, since they are counted in different stores.
Why are the reviews mixed?
Two things come up again and again. First, sales figures are estimates: Amazon does not publish unit sales, so every tool converts Best Sellers Rank into numbers with its own model, and those numbers can be well off for individual books. Second, the extension depends on Amazon’s page layout; when Amazon changes it, the extension can stop showing data until it is updated. Treat the figures as a way to compare niches, not as a forecast of your own income.
Does it cost money?
The extension needs a Book Bolt account, and Book Bolt sells paid plans with more research tools. What is included for free changes over time, so check Book Bolt’s pricing page before you build your workflow around it.
Frequently asked questions
Are there alternatives?
Yes. Several KDP research tools offer similar rank-based estimates, and Amazon’s own Best Sellers lists and search suggestions are a free starting point.
Is using KDP Spy against Amazon’s rules?
It reads public product pages while you browse; it does not change your listings or your KDP account.
KDP Spy is a quick way to size up a book niche from Amazon’s own search results. Use its numbers to compare ideas, double-check promising niches by hand, and remember that estimated sales are only estimates. For more on choosing safe browser add-ons, see our guide to installing Chrome extensions.