> Amazon also had your entire purchasing history, and its servers could instantly compute recommendations you would be likely to accept.

I wish. I have long been surprised at how poor Amazon recommendations are given that they have a large pool of actionable data: things I have actually purchased. What else could be a stronger signal?

I have even informed them which purchases not to use as a basis for recommendations, or when recommendations they have made are not interesting (at least, those that are made from my page while signed in, no ability to do so with the emails they send).

I preorder plenty of technical books, so Amazon could easily suck more money out of me if they kept me informed on up and coming books by authors I have purchased from, or in the specific area I purchase in. Instead, I seem to get weekly emails about the latest "popular" books in a very general area (e.g. programming).