No matter how beautiful the cover, nor how compelling the blurb, if the first chapter doesn't resonate with the reader they're not going to buy the book.* If it's a typo-strewn train wreck they're even less likely to shell out for a copy.**
Having said that, there are compelling reasons to post a sample:
If people read the sample and can't stand it, they're unlikely to hunt down every online listing and leave scathing reviews. Now imagine they shelled out 5, 10 or 20 bucks on an ebook or paperback, sight unseen.
If they read the sample and enjoy it, you've probably made a sale.
A poor cover and/or title can give people the wrong impression. If they get past those and read the first chapter, you've been given a second chance to impress.
Same with a wonky blurb. If it makes the novel sound like something it isn't, a sample chapter or two can undo the damage.
So how much of a sample should you post? Enough to give them an idea, not so much that they're wondering when it's going to end. (Especially if the sample has to be read on a computer screen.) I think two or three chapters is plenty.
So what prompted my thoughts on sample chapters? I've just been tussling with this very subject, that's what. I've been thinking to myself, "If I've written a lousy book I'll just try and sell it on the cover art, the blurb and the press release." I've also been thinking "What if the emperor has no clothes? If I upload a sample and everyone thinks it's crap, I'll be a laughing stock. And nude!"
Sooner or later you have to take the risk: Here are the first two chapters of Hal Junior: The Secret Signal.
And relax, I'll make sure I'm wearing my space jocks.
* Unless it's a gift for someone else.
** Unless it's a gift for someone they don't like.
Simon Haynes is the author of the Hal Spacejock and Hal Junior series (Amazon / Smashwords / other formats)