As I understand it we all see the same adverts from Amazon ..... Yes they are "Targeted" but not to individual members. Amazon's "Bots" follow the trend of postings, seeking triggers that will show the sort of thing that we are posting about in general and selecting books that they think we may be likely to buy.
Every thread attracts different comments from us and this in turn triggers the adverts deemed most suitable.
The number of Snoopy related books and products that pop up shows that they are using "Key words and Phrases" to decide what we should be offered.
It all operates much the same way as using a "Loyalty Card" in Tesco, Sainsbugs etc enables them to target the offers and discount coupons they send you.
I don't think it is anything more sinister than that.