They weren’t attacking the author. It’s a valid argument regardless of the cost of the book. Free doesn’t mean you can’t criticize anything.

There was an accusatory, hostile tone that I personally found off-putting and not worthy of an HN discussion.

asicsp’s comment[1] was much more useful and neutral.

1: https://news.ycombinator.com/item?id=26074429

He stated his experiences with mathematics and computer sciences books and provides arguments for why he thinks this is not a good approach. I really can't see where he is being hostile or accusatory?

Tone policing on a textual medium isn't helpful either.