Plugins are the best way to extend GitBook functionalities (ebook and website). There exist plugins to do a lot of things: bring math formulas display support, track visits using Google Analytic, ...
Plugins can be easily searched on plugins.gitbook.com.
Once you find a plugin that you want to install, you need to add it to your book.json:
{
"plugins": ["myPlugin", "anotherPlugin"]
}
You can also specify a specific version using: "[email protected]", this is usefull when you're using an outdated version of GitBook.
If you're building your book locally, download and prepare plugins simply by running: gitbook install.