Amazon tends to recommend things I like and the majority of the purchases have been because of their recommendations (good job Amazon, your software is doing it's job, raising sales). I think books and music are different though. Books have well defined categories. If I buy a pop-psych book, say "Blink", and then I am recommended "Peak: Secrets from the New Science of Expertise" it's pretty much going to be very likely I buy that too if I am interested in that subject.

If however I want recommendations for new Metal music, and my previous selection was Metallica then you play me some Megadeth, I am going to hate it and not be interested in it at all!

Agreed. "Oh, you like Horror? Have you heard of Stephen King?"

I've completely given up on goodreads for recommendations and just google "best <insert genre> books 2017" now and usually can find some good lists.