You can use install it via NPM:

$ npm install gitbook-plugin-disqus

And use it for your book with:

$ gitbook build ./ --plugins=disqus

You can set the Disqus shortname using the plugins configuration (command line option: --pluginsConfig) with the following content:

{
    "disqus": {
        "shortName": "XXXXXXX"
    }
}