Last reviewed by Robert Prime — May 2026
Introduction
The single most expensive mistake in self-publishing is writing 90,000 words in a genre nobody reads. Five hours of market research before chapter one prevents it. This is the validation system used by the indie authors who actually sell books.
It's not glamorous. It's not the bit aspiring authors talk about on Twitter. But it's the difference between launching into a hungry audience and launching into silence.
Why this matters more for indies than for traditionally-published authors
Traditional publishers do market research before they buy a book. They check comps, look at the genre's recent performance, evaluate the author's platform. Acquisitions editors are paid to make this judgement.
When you self-publish, you are the acquisitions editor. Skipping the research is skipping the most important part of the editor's job.
The good news: Amazon makes this research accessible. Almost all the data you need is in public Amazon listings — you just need to know where to look.
The minimum viable validation (5 hours)
A first-time author validating a book idea should be able to answer five questions before writing chapter one:
- What genre is this book? Not "fiction" — the specific Amazon category. "Police procedural" not "thriller". "Dark academia" not "fantasy".
- Who are the 5 closest comp titles? Five successful indie or trad titles in the same exact sub-genre, in the same era, at a similar reading level.
- Are these comps selling? Amazon Best Seller Rank (BSR) translates roughly to monthly sales. A BSR of 5,000 in Kindle ≈ 250+ sales/month. A BSR of 100,000 ≈ 5 sales/month.
- How many reviews do they have, and how fast? A book with 200 reviews in 12 months has velocity. A book with 50 reviews in 5 years doesn't.
- Is it a series? Standalone successes are rare in indie. If the top comps are all book 1 of a series, you should plan a series.
If you can answer all five with positive signal — proceed. If three or more are weak, you're either in the wrong genre or your idea needs reshaping.
How to find your comps
Step 1: identify the specific Amazon category. Go to amazon.co.uk → Books → Browse Kindle Store. Drill down through the category tree until you find the most specific category your book fits. Note the path (e.g. Kindle Store → Kindle eBooks → Fiction → Mystery, Thriller & Suspense → Mystery → Cozy → Animal Cozy Mysteries).
Step 2: list the top 20-50 books in that category. Just the title and BSR.
Step 3: filter to genuine comps. Same era (contemporary, historical, set in current day). Same tone (light cosy vs dark literary). Same reading level. Same word count range. Same protagonist demographic (an 80-year-old retired detective vs a 25-year-old amateur sleuth are different markets).
Step 4: cross-check on Goodreads. Goodreads shelves and "Readers also enjoyed" reveal connection patterns Amazon doesn't.
Step 5: pick 5 comps. Three trad-published, two indie ideally — that mix tells you what the genre's quality bar is and what indie can realistically achieve.
Translating BSR to sales
Approximate monthly Kindle sales by BSR (2026 figures, varies by category):
| BSR | Approximate monthly Kindle sales |
|---|---|
| 100 | 3,000+ |
| 1,000 | 800-1,500 |
| 5,000 | 200-400 |
| 10,000 | 100-200 |
| 25,000 | 30-60 |
| 50,000 | 12-25 |
| 100,000 | 4-8 |
| 500,000 | <1 |
If your 5 comps are sitting at BSR 50,000-200,000, the category is alive but small. If they're at BSR 5,000-20,000, the category is healthy. If they're at BSR <2,000, the category is competitive — you'll need to compete on quality and marketing.
If your 5 comps are at BSR >500,000 across the board: the genre is dead or you've picked the wrong comps.
Review velocity as a signal
A book with 200 reviews collected in 12 months has roughly 20,000-40,000 sales (1-2% of buyers review). That's healthy.
A book with 200 reviews collected over 6 years has 20,000 sales over 6 years — possible, but the velocity is slow.
Velocity matters because it's a signal about the genre's energy now, not 5 years ago.
Free research tools
- Amazon Best Seller pages — every category has one. Tells you the top 100 in any sub-genre.
- Goodreads "Listopia" — reader-curated lists for hyper-specific genres ("Best Cozy Mysteries with Cats", "Best Slow-Burn Sapphic Fantasy").
- /r/booksuggestions and genre-specific subreddits — what readers ask for.
- TikTok #BookTok hashtag — what's trending in the social-media-driven segments of indie publishing.
- Amazon's "Customers also bought" carousel on any successful comp — opens up the network of related titles.
Paid research tools
- Publisher Rocket (£99 one-off) — calculates BSR-to-sales, finds keyword competition, shows category competition. The single most-used indie research tool.
- K-lytics genre reports (£20-£40 each) — deep reports on specific genres' sales, prices, and trends.
- Bookstat / Stair Steps Books — pro-level data, expensive (£100+/month).
For a first-time author validating one book: Publisher Rocket pays back its £99 in saved time.
The "comp title" framing
For each of your 5 comps, write one paragraph:
Title — Author — BSR — Reviews — Price — Series? — Why this is a comp for me — What I'd do differently
A finished comp document is 5 paragraphs. It tells you:
- What price range readers expect (£0.99-£4.99 for indie ebook usually)
- What the cover style is (study the thumbnails)
- What the blurb pattern is
- Whether the market wants a series
- Where the quality bar sits
When to pivot
Three signals that your idea needs to change:
- No comps exist. If you can't find 5 comps in your genre at reasonable BSR, your genre is dead, fragmented, or so niche it's not commercially viable. Either pick a more commercial adjacent genre or accept it'll be a passion project.
- All comps are 10+ years old. The genre had its moment. It's not where Amazon's algorithm and readers' attention are now.
- Successful comps look very different from your idea. If your book is light cosy mystery but every successful comp is dark literary suspense, you've misidentified your genre — or your book needs to change.
UK-specific considerations
- Amazon UK has smaller per-genre volumes than Amazon US. A book that's #50 on Amazon US in cozy mystery might be #15 on Amazon UK — but with fewer sales. Validate against the marketplace where you'll launch.
- British genres are sometimes US-misnamed. "Country mystery" in the US ≈ "village mystery" in UK terms. Get the local genre vocabulary right.
- UK readers index higher on print than US readers. A successful UK indie series usually has print editions, not just ebook.
- UK-set fiction sells well in both markets but US-set fiction by UK authors is harder to position. Be aware.
What not to do
- Don't research only your idea's strengths. Look for weakness signals too. If every comp has a 4.6+ star average but yours is "challenging literary prose", you may be in the wrong market.
- Don't validate only via Twitter/Reddit. Vocal online minorities don't represent buying audiences. Amazon BSR and review velocity do.
- Don't write the book first and validate after. This is the most expensive mistake. Five hours up front prevents 500 hours of regret.
- Don't use exact-title competition. "Is there another book called X?" is the wrong question. The right question is "does this kind of book sell?"
Common mistakes
- Picking comps too aspirational. If your comps are Lee Child and J.K. Rowling, you're picking outliers. Pick mid-tier comps — BSR 2,000-30,000 — that represent realistic outcomes.
- Ignoring series signals. If every comp is "book 1 of 5", and you're planning a standalone, the genre is telling you something.
- Conflating "I'd buy this" with "the market would buy this". Personal taste ≠ market validation.
- Skipping the research because "my book is unique". Unique books still need readers, and readers are organised by genre.
The bottom line
Five hours up front. Five comp titles. Five questions answered. If the signal is positive, you have a market and a genre. If it isn't, you save yourself months of writing the wrong book.
Validation isn't about killing creativity — it's about pointing creativity at where readers actually are.
Frequently asked questions
What if my idea is genuinely original and has no comps?
Then it has no readers either. Truly novel categories take 5-10 years to build an audience. As an indie author trying to earn money, work in established categories. Save the original idea for book 5 once you have an audience.
How current do comps need to be?
Comps published in the last 3-5 years are best. Anything older may reflect a market that's moved on.
Should I validate non-fiction the same way?
Yes — comp titles, BSR, review velocity. Non-fiction has the advantage of measurable problem-solving: search Amazon for the exact reader question your book answers.
What if Publisher Rocket says my keywords are too competitive?
That's useful information. Either find less competitive long-tail keywords or accept you'll need to launch with paid ads from day one.
Can I validate without spending any money?
Yes — Amazon Best Seller pages + Goodreads Listopia + manual research. Publisher Rocket accelerates it but isn't essential.
