There are at least four books so far in the "unofficial" series, and if you believe that the companies involved wouldn't copyright strike it if they could, that's protected by the first amendment.

https://www.mugglenet.com/site/publications/

You can also find unofficial and unauthorized books in other settings, such as Minecraft, but you can argue that it's explicitly permitted as seen here: https://www.minecraft.net/en-us/article/commercial-use-guide... - but you can ignore those in the USA in many cases - here's one already: https://www.amazon.com/Minecraft-Monarchies-UnOfficial-Story...

Okay, it's not "skirting incredibly close" to write those reference books, because they are in no way a substitute for the underlying fiction, which is why they would probably be considered a fair use.

>There are at least four books so far in the "unofficial" series, and if you believe that the companies involved wouldn't copyright strike it if they could, that's protected by the first amendment.

You have no idea what you are talking about. The first amendment isn't a defense to copyright infringement. transformative fair use with a de minimis copying is.

I was saying that if you believed that the companies involved wouldn't strike these if they could, that belief is covered by the first amendment freedom of religion.

You said they were strikingly close to the line, they aren't. The implicit suggestion being one can just write harry potter fiction by referring to harry potter facts, you can't!