I remember a friend of mine was invited to write a chapter for a Java book for a very popular, very successful and prestigious publisher; the book wasn't even out yet but had a page on amazon and they made him ask all his friends and family to write a positive and "believable" review like "it is an excellent introduction to blablabla, this and that thing could be better/more detailed but overall best book ever!!!111"

They justified that by "well, yea those are sort of fake review but everyone does it so it is ok"