Dmitry, nobody (or nobody sane) is suggesting that there is actually a conspiracy of white people to suppress black people.
But the point about concepts like "white privilege" is that it doesn't *require* an active conspiracy. Society in some countries has developed in such a way that black people are frequently at a disadvantage to white people.
Take the books example. Of course, there is no rule that black people can't write books, and many black people *do* write books. And of course, we would expect there to be more white authors than black ones in the USA, because there are far more white people.
But still...black authors are, I'm sure, disproportionately few.
Why? Well, for one thing, average black household income is lower than average white household income. This may translate into fewer educational opportunities, and since it tends to be well-educated people who write books, that would then mean fewer black authors.
Now, that's probably only part of the explanation. (I suspect another part is that black people are perceived by some publishers as writing "black books" that will only be of interest to the black market, whereas nobody presumes that only white readers will purchase books by white people.)
But the point here is that the paucity of books by black authors is not an issue about bookstores or publishers *specifically*; it's a symptom of a more general disadvantage.
And while saying that "black people are not prevented from writing books" is technically true, it's missing the point: that the way American society and many other western societies are set up, writing books is made *harder* for black people than for white ones. No one individual, no one rule, is responsible for this, which is one of the reasons it's been so difficult to change.
Of course, averages are only averages, and there are many exceptions. There are plenty of wealthy, successful, well-educated black people and plenty of poor, ill-educated white people. So nobody is sensibly arguing that every single limitation or advantage applies in every single case.
However, it is undeniable that on all sorts of hard and soft indicators relating to poverty, education, employment, visibility in the media, political power, and so on, black people are disadvantaged or under-represented.