Assuming that more than one person can win this auction (no-one said there'd only be one advert in back of a given book, right?), this is actually an attempt at what economists call price discrimination, which is a type of selling which absolutely favours the seller over the buyer.
If you go to your supermarket, you're given a set price for a bottle of milk, whether you're a millionaire or penniless. The supermarket would love to be able to charge the millionaire more, but the means of selling makes that impossible. With an auction, only the very highest bid succeeds, with the maximum the buyer is willing to stump up. PA's auction appears to be a set of bids where the seller can decide how many can succeed. It scoops off the maximum amount of revenue from a number of customers decided by the seller. That being so, it promises to provide a much higher level of revenue than simply charging advertising rates.
Ethically, it's wrong on so many levels it's physically painful, but as an exercise in revenue gathering, it has a brutal charm.