Literary agent David Fugate of LaunchBooks Literary Agency has written,
looking back at my PublishersMarketplace search, I noted that 29 deals were listed where the book had been previously self-published. That sounds like a decent number until you realize that more than 30,000 deals have been reported there over the last 2 years.So out of every thousand deals reported by Publishers' Marketplace over the last two years, only one concerned a book which had previously been self-published: a statistic which effectively flattens the argument that self-publishing provides a good route to mainstream success.
I'd really like to find out how many books were self-published in that same time-frame, and how many of them went on to be published by mainstream publishers: but it's proved impossible to find anything verifiable and hype-free (if anyone knows where I can get such statistics for free, I'd love to have them). Because if we had those figures we'd be able to see just how likely it is for books to cross over from self-publishing to mainstream, and possibly how fast this trend is increasing.
None of this is meant to imply that self-published books can't be successful without making that transition: of course they can, under the right circumstances. Mr Fugate goes on to write,
Does that mean I think you shouldn’t self-publish your book? Not necessarily. There are circumstances where self-publishing - especially when taking advantage of the speed and ease of print on demand - makes a great deal of sense (generally when speed to market is very important and where the author has a significant marketing platform to draw upon).
Bolding mine. If you're going to self-publish, you're going to have to do a lot of marketing and promotion in order to see any significant sales; and unless you go on to make those significant sales, you're no more likely to find yourself a mainstream publisher than anyone else who has written a book. Unless your book is stonkingly good, which is, of course, a whole different discussion.