The most accurate statement I can make about the growth rate of the indie/author segment of the publishing industry is that it is nebulous at best. This part of the industry is so fuzzy that it can't be counted, but that hasn't stopped anyone from trying.
Digital Book World, for example, summarized a press release yesterday which assumed that ISBNs were an accurate measure of growth. The pr was from Bowker, and under the title "Self-Publishing Maturing, Up 17% Last Year in the U.S. ", DBW wrote:
The self-publishing market is entering a new stage of maturity after an initial boom several years ago, according to Bowker’s latest analysis of ISBN registrations in the U.S. from 2008 through 2013.
To be sure, not all self-published authors obtain ISBNs for their work, but among those that have done so to date, their output of titles is increasing. The number of ISBNs registered in 2013 rose nearly 17% from the previous year.
That growth comes not from ebooks, which actually dropped 1.6% during that period, but from print titles, which rose 29%.
Bowker researchers conclude that the self-publishing market is “stabilizing as the trend of self-publisher as business-owner, rather than writer only, continues.”
I will freely admit that I don't have a clue how to measure the growth of the indie author, but one thing I can tell you is that ISBNs aren't a valid measure. ISBNs have about as much relation to measuring the growth of the publishing industry as standardized test scores have to measuring academic progress - in other words, very little.
Sure, many authors get ISBNs for their books, but there are also an unknown number of holdouts, including Hugh Howey, who insists they are not necessary.
As one commenter put it:
It’s far higher than 17%. Most self-pubbers either use free, POD assigned ISBNs or, for ebooks, don’t use any at all. Also, many self-pubbers use publishing house names, so there’s no real way to know if they’re self-published.
And that's not the only point of inaccuracy; even what data Bowker does have about ISBN use may not be accurate. Another commenter on that DBW post noted that " many indie authors use the free ISBN provide by CreateSpace in their ebooks, contrary to the clear guidelines that ebooks and print books require a separate ISBN each", attributing this move to the cost of ISBNs. This goes against Bowker's rules, but he pointed out that even Amazon assigns a single ISBN to both a print and digital edition.
To put it simply, no one knows the complete state of the publishing industry, and anyone who claims to do so is selling you something. (In the case of Bowker, they're giving it away; the report is free.)
image by Neon Tommy