"I think it would be nice to have an actual "book", with community-composed chapters. They would follow the same topics listed here, but with actual cohesive explanations and background, rather than just a list of resources." - hmm... PR welcome :) Prove yourself man.
I do it only for stars... really man? Creating and maintaining such a list is not a simple thing. It's not awesome or other shit in the name.
This is a collection of my short notes, one-liners, useful links, tools and more. I added it to gh, to share with others and get other interesting things. Not for stars! The amount of them is obviously nice but not crucial.
"and most of the hardening seems questionable at best." - Don't do this, please add rationale, not bullshit.
"Some of the headers that is suggested have some implications that aren't really explained at all. Like HSTS including sub-domains." - This repo also contain "Force all connections over TLS". However, I understand your attention.
"And you don't set the `default_server` as this document suggest. `default_server` is a parameter to the `listen`-directive. The only reason the docs might work is because the first server-block defined, when no-one is defined as `default_server`, becomes the default server." - You're right, your suggestion is very well (it's also from Nginx official handbook) and rationale, thanks for this! I receive this criticism, My mistake.