Perhaps, but one needed to survive and thrive in business.

No freaking way. No one says you have to cooperate with everybody. In fact, creating an alliance of cooperators to stand in opposition to an uncooperative entity is a fairly effective tactic.

Because it is a undisclosed conflict of interest.Imagine you go with a friend to a bookstore , and he recommends to you several books. Since you seem to trust your friend and you have the money, you buy the books. Unbenknownst to you, your friend receives a 5% commission, and he never disclosed it to you. Same principle. It is a whole different game if he mentions this to you in advance, since now you consider your friend a as an interested part and you will consider his recommendations more carefully.

I would see a conflict if it was selective as to which book. But this gives no preference to one book over another.

So I'm still not getting it. I see no conflict.

So? He can overhype all the books without giving preference to one. He can pontificate about how important is to read books, how all the geniuses at HN read these very important books and so on.He only needs to maximize the money spent on the books, not the selling of a particular one. He is advertising beers (and get money from all of them) not just Budweiser.

Except this page discloses it...

It's possibly more the case that he'd rather just dip in and out as and when it pleases him. No harm in that of course, I know plenty of people in the same situation who are perfectly capable but wouldn't dream of doing it full time.