I’m assuming it means mentioned as a post. I know I’ve personally seen Cal Newport’s, Deep Work mentioned in the comments at least a dozen times.
The thing all those comments have in common? They contain Amazon links.
I prefer this list: https://hackernewsbooks.com/
All links to Amazon, Safaribooks and O'Reilly get extracted once a week from Hacker News posts, make sure they are indeed books and then rank them based on how often they are mentioned and the karma of the user.
Ahh. I guess that's it - people don't bother giving Amazon/other links for well-known books. Or sometimes the book has its own website.
Also, that site makes money by linking to Amazon. That seems a bit dodgy to me!
Hmm is it really that hard to extract book titles that aren't links to Amazon/publishers? C'mon hackers! :-)