I've been an Amazon customer since the early 2000s now. Though I suspect I'm not what they consider a good customer, because I only buy books and hardly anything else there. I don#t have Prime either. I think I may have bought six or seven non-book products at Amazon in fifteen years or so. And every time, I'm so inundated with ads for non-book products (no, the fact that I ordered vacuum cleaner bags from you does not mean that I am even remotely interested in a breadmaker
) that I'm not likely to order anything that's not a book there again. Because I don't mind receiving ads for books.
However, Amazon's recommendation e-mails have been getting steadily worse for years. I used to get e-mails recommending books very similar to what I bought before. It got messed up on occasion, e.g. when I ordered a motorbike book for my Dad and was inundated with ads for motorbike manuals, or when I bought a linguistics textbook and suddenly got ads for Hungarian grammar books. But usually, the recommendation e-mails were on point. Until they suddenly weren't anymore.
For example, Amazon keeps sending my new release announcements for a very prolific author of East Friesian mysteries. I've never bought a single book by that author, don't follow them and don't even read East Friesian mysteries (and I buy more English than German language books). But Amazon feels the need to inform me about every single one of this author's new releases.
I do read science fiction and fantasy. However, I don't read every subgenre. And for a long time, I got recommendation e-mails for books in the subgenres I read. But then I started getting recommendations for books that are SFF, but not in subgenres I read. Interestingly, they were mostly from the same publisher, a publisher I rarely buy from except for two authors, because their offerings are not to my taste. However, I never got ads for the two authors I actually buy, even if they have new books out, but for only authors I don't buy. Once I even got an e-mail focussed only an author's new release. Now I have never bought a single book by that author, dislike their writing, have never bought anything remotely similar and have been personally insulted by that author (which Amazon cannot know, but which explains my reaction upon getting that e-mail
). In this case, I suspect the traditional publisher paid Amazon for advertising their books to anyone even remotely interested in SFF.
Anyway, these past few years I have been buying mainly at Thalia (print) and Kobo (e-book) and only shop at Amazon for books that are exclusive, POD books, which are very difficult to get elsewhere in Germany, and certain small press and university press books the other stores don't carry.