While I think the moderators do a pretty decent job, there have been plenty of claims of favoritism toward the advertisers. Even if the moderators try to be fair, they may still have an affinity for the books. Also, Shrink can pick moderators that are easily swayed by the types of arguments that the books make. (Like the way Dems and Repubs fight for judges that believe in different interpretations of the constitution.)
That said, I think the group that advertises here is a pretty strong group of books. Past "bad books" have gotten shot down by the players and left. Other probabably expected biased treatment in their favor, and left when they found out how vocal the forum members can be. Sort of a "Darwin Natural Selection" that has weeded out the bad books and left the RX will more solid advertisers.
At this point, most of the bias in favor of the books is due to the history/trust the moderators have with them, compared to a player that they are just meeting when a complaint comes in. That is inevitable. However, if a book tries to make an argument opposite to a position they once took, the mods can catch them being inconsistent.