In a blacklist something is completely blocked. In a whitelist something is absolutely allowed. In a graylist you may be allowed after a certain number of attempts, or after completing some sort of task, etc. It's kind of like a soft block.
In this scenario the website detects the link is coming from HN and "graylists" your visit to the website. You are redirected to a page that explains he's trying to sell a book and people coming from this site just use his bandwidth and never pay. But the website explains if you don't want to pay you can still get to the book by browsing to the website directly -- as opposed to being redirected there from HN.