(Like "Ask HN: Math books that made you significantly better at math?" https://news.ycombinator.com/item?id=34439828 and "Ask HN: What are the most eye-opening textbooks you have ever read?" https://news.ycombinator.com/item?id=35929112 and "Ask HN: Sci-Fi Book Recommendations" https://news.ycombinator.com/item?id=35052757)
As such, the selection process here will be biased towards the kinds of 'Ask HN' posts that can get voted highly (to be visible, to attract many comments), and within those, these are the most frequently mentioned books.
(Not that there's anything wrong with that, but it's always a good idea to pay attention to the selection criteria, as selection effects are everywhere. For example, it would also make sense to exclude such threads, to get a more representative idea of what books HN comments tend to talk about in natural contexts.)