Most authors will appreciate where I come from with this. You slave God-knows-how-many-hours, weeks, months (years) on a book. Probably in your spare time, whilst still earning a "regular" living. You finally get an agent, then a publisher interested, after numerous re-drafts of your pitch, your content and -- in my case -- paying thousands of £/$ for flight costs, hotels and research *out of your own pocket*.
You then (in my case, for the book Homeland) have to hire your own editor, because your publisher refuses/cannot (apparently) afford the time/resources/unpaid intern to actually edit your work. You try your best, with the limited cash (overdraft) you have available, to put out as good a work as you can. You stand back, exhausted, and marvel it has come this far. You've given birth to something unique, that doesn't simply parrot what's already out there; which is not a celeb ghostwritten piece of crap; and which you also have to have extensively checked by researchers and legal sources to ensure no libel.
Then the fear sets in.
What if no-one will notice the book when it comes out? Will it lead to more work? Has it been worth the slog, the damage to your relationships, the stress, the threats passed your way (how many times have Internet weirdoes and sad violent guys sought to have me followed ...) -- and will it be properly reviewed?
You then (if lucky) get your two week window of fame. The PR ladies at the publisher get you interviews on a few radio stations; you try and get on Richard and Judy, just missing it (too intellectual/not enough undercover violence!); you pen articles; pay for your own internet ad campaign (Google); and generally stick another huge amount of effort into getting it noticed. The reviewers (who, after local radio stations, are more or less the only people your publisher has to ensure *any* marketing at all) then ignore/maul/promise to review (but don't)/or sometimes give nice reviews to your work. I could debate the quality of professional reviewers -- I'm one myself -- let's just say there are some very good ones, and then there are those like the ladies who go on to Radio 4 and write in The Times, who clearly don't read the entire thing and give some liberal hogwash opinion which says more about their own worldview and prejudices than your own work. Time and time again I could tell who had, and had not, read the work by the tone of their comments. It is why when I review, if I don't like something I substantiate it, or give the writer the benefit of the doubt ("it's not to my taste, but I can see x y z liking it").
Be fair is my guiding principle.
But then all goes quiet. If you don't have a weekly column, face on Newsnight Review, making documentaries or generally some other "star" quality, your tome disappears among the 120,000+ published each year. Your publisher has no budget left to market and is already promoting the next work which, unbelievably, has almost the same title as your own yet is concocted trash. The publisher has no budget to promote your work each time something occurs in the news. The big fat obvious truth is that you're on your own. You're a one man band and need to promote that work yourself. In fact, if you could hire a printers and do deals with vendors, you could probably sell it almost as well as they can. That just leaves Amazon.
Amazon operates a readers' review system: people give stars and comments, from 1-5 (1 = bad, 5 = good). Many writers try and stuff their friends in, awarding suspicious 5 stars. But as time goes on, you're left with -- as my old GMTV friends called it - "the nuts and sluts". Having written a book on the extreme Right, I only get white supremacist nutters coming along these days. They uniformly give the work 1 star, which clearly has less to do with any writing or research quality (or even if they fully read it), more to do with their own obvious bias and political views.
I don't object to people not liking something, but I do object to liars, falsehoods and slander. Left at the mercy of these half-hinged cranks, every future reader sees such comments and is left with a plethora of negative reviews which they may, just may, think are fair and accurate comments. Like I said, as a professional reviewer I rarely stoop to such lows and underhand tactics, even if I really don't like something.
But if publishers aren't paying for marketing, and the author is relying on word of mouth, the tyranny of Amazon represents a powerful force in all future sales. And it is those who "shout" loudest on the Internet that, sadly, seem to get heard.
(In the interests of fairness, you can see some examples of this "he who shouts loudest" from the reviews on my Amazon.com page. Note that the lead "negative" reviewer is a self-proclaimed UFO expert ... How come Jon Ronson gets to stare at goats and they love him, whilst my chaps just hate me ... ^^)