Personally, I think Amazon’s recommendation quality has declined precipitously over the last year or so. I find the way they do the recommendations page now to be totally worthless. Often, more than half the books recommended are different formats of books I’ve already purchased from them. The old format, by genre etc., was much more usable. Goodreads does a better job, but it’s far from perfect, e.g., mystery recommendations under science, etc. In reality, I frequently get better recommendations by searching books on Hacker News.