I'm really confused about how to use plugins.

[Ir1d]

The help doc didn't help very much here.

I added this into my book.json:

{
   plugins: ["katex"]
}

but the build failed, so I changed into this:

{
    "plugins": ["katex"]
}

again it did not work, telling me that

info: load plugin katex ....ERROR
Error: Error loading plugins: katex. Run 'gitbook install' to install plugins from NPM.

So what exactly should I be doing to enable MathJax for my doc?

Thanks again for this great system.

[SamyPesse]

This plugin is mathjax, not katex.

You did the right thing, but like the message is telling you, you need to run gitbook install to download plugins.