Hi Oliver,
Interesting and frustrating indeed.
I don't think it is a bug, but my thoughts would be that it is some sort of association as you suspect between the two sites in Google's eyes. I didn't look a lot but your site does seem quite similar to amazon in terms of product offerings and I am wondering if in all the data available on click throughs etc that google has maybe a significant percentage of searchers end up at both sites quite often and therefore the link has been made. I would think perhaps the two brands are also mentioned quite a lot together and even linked to together quite often? Its not I guess that unreasonable if that is the case that google would choose to show these results, and amazon is amazon with the associated domain authority etc that goes along with it so might often get pushed to the top.
In regards the search queries you mention, do you think people do a lot of searches for the name of a book + the brand priceminister? It does not seem that intuitive that they would, although I might be wrong! If the searches are not happening that often, and since amazon is ranking highly anyway for the titles of many books, then maybe the book title phrases are simply out weighing the added brand keyword if you see what I mean. The google plus example is indeed a bit odd, but that being said if you search for 'priceminister google+' which I seem to recall is the 'official' way to search for g+ pages then you are front and center.
Not that much help in terms of what to do about it, maybe someone else will have a good idea!