On this blog last month we unpacked some data presented by Nielsen at Digital Book World on the performance of the ebook market. We discovered that it was possible to use the data that Nielsen presented to guess at the size of the US publishing market, but with one major caveat. Nielsen only counts books sold with ISBNs, so doesn’t include a significant part of the self-published (or indie published) market that sells books without ISBNs.

At the time we noted that this meant a big chunk of the book market was essentially invisible and we had no way of guessing how big it was. Now, thanks to the work of indie author Hugh Howey’s Author Earnings report, it might just be possible to guess at a true size for the ebook market in the US – for adult fiction and non-fiction at least.

We’ve discussed earlier editions of Author Earnings data in some depth on this blog. It’s a useful source of data if you’re interested in tracking how the efforts of traditional publishers stack up against the self-published or indie published sector on Amazon at least. There are some lasting questions as to whether its method, which extrapolates upwards from the Top 10,000 performing titles on Kindle, is fully representative of overall market trends. After all, by concentrating his analysis on just 120,000 out of a total of 3,285,000 available titles, Howey is essentially basing his conclusions on the whole market on the performance of just 4% of ebooks. He does go on to claim that these titles take almost 50% of daily ebook sales, but without citing the source of this information, or showing how he used available data to arrive at this figure, it’s difficult to verify it.

Nevertheless, this round of Author Earnings data does contain one piece of very interesting information. Howey and his unnamed helpers picked through every one of the 120,000 ebooks whose performance they tracked to see which did and didn’t have ISBNs. They discovered that only 70% of these ebooks did have ISBNs. If we assume that this 4% sample is representative of the 100% this suggests that the size of the ebook market is underestimated by 30%.

Nielsen Market Figures

To recap on our earlier blog post, Nielsen indicated that in 2014, US publishers sold almost 142.5 million adult fiction titles in the three major print formats (hardback, trade paperback and mass market paperback) and 132 million ebooks (across all platforms).

Therefore according to Nielsen adult fiction accounts for 275.5 million book sales, with ebooks taking close to half (48%) of that market.

Fiction Book Sales (exc non-ISBN)

At the time we noted that these figures showed the industry had gone from selling 212.5 million adult fiction titles in 2008 to 275.5 million in 2014.

This meant that the ebook revolution had driven overall market growth (in volume at least) of 30% in the adult fiction category.

The story is very different, however, when you look at the non-fiction market. According to Nielsen data the publishing industry sold 267 million books across all major print formats in 2014, but only 37 million ebooks.

Non-fiction (exc non ISBN)

This put the size of the total adult non-fiction market at 304 million books and ebooks’ share of it at 12%. This represents growth of only 1% since 2008, so at best the digital revolution has enabled the non-fiction sector to stand still.

How Author Earnings estimates would change Nielsen’s market figures

Let’s assume for the moment that it’s possible to take that Author Earnings’ 70% number is accurate and representative. Let’s also assume that sales of those ‘invisible’ 30% of ebooks even out on aggregate and don’t contain one runaway hit with the power to skew the numbers in the style of Fifty Shades of Grey.

An ebook market 30% larger than Nielsens’s would have sold 219.7 million adult ebooks in 2014. That would work out as 48 million adult non-fiction and 171.7 million for adult fiction. Again, that’s assuming non-ISBN ebooks sell in the same proportions as ISBN ebooks.

Fiction book sales (inc non-ISBN)

This would mean the adult fiction market, comprising both published and self-published physical books and ebooks, amounted to 314.2 million units. This would give ebooks a 54% share of the adult fiction market. It also suggests that the contribution of traditionally and self-published ebooks has grown the adult fiction market by 48% growth since 2008.

Non fiction (inc non-ISBN)

Counting non-ISBN ebooks, the non-fiction market in 2014 was 315 million books, compared to 300 million in 2008. While this number is larger, it still represents only 1.5% growth since 2008. Once again, while digital publishing has demonstrably changed how people buy fiction, it has yet to crack the non-fiction market.


Howey is right when he asserts that not counting sales of books without ISBNs significantly affects how we assess the total size of the ebook market. This ‘invisible’ 30% of ebooks would push the digital publishing sector back into growth at a time when talk of plateau risks talking the book business down. It also explains why the data emerging from companies like Nielsen appears to track a slowdown in the ebook market. As indie published authors abandon ISBNs (or never adopt them in the first place) their sales drop off the edge of the map.

fiction and non-fiction inc non-ISBN

If Howey’s assertions are correct, and his projections are accurate, Nielsen’s current best estimates undervalue the total adult fiction and non-fiction book business by around 8.5%.

What is not clear, however, is how independent bodies like Nielsen could ever hope to track this market accurately without a structured data standard like the ISBN to identify individual books and count their sales. Until someone invents an alternative acceptable both to the traditional publishing industry and the indie publishing community the market will continue to be invisible.