This is getting really nitpicky, but it's below "the fold." So it might as well not be there for some users.

But getting people to buy the book might not be the primary goal of the website, or the home page. It's probably to show off the reviews and teardowns, which it does a great job of. I think it's perfectly acceptable to have the secondary purpose of the site, buying the book, a bit out of the way.

It's been mentioned that it's below the fold, but it's also worth noting that it's not even on the linked page.

The article page linked to from HN is "http://www.useronboard.com/onboarding-teardowns/", which has zero calls to action on it natively. If I go back to home, yes, I see the button, below the fold, presumably it exists on other pages as well, but it is definitely not on the linked page, which had me questioning why I couldn't find it (and spending an inordinately long time looking for it).

it's below the fold. dunno if the parent noticed it or not..