Add Google Analytics to gitbook
npm install gitbook-plugin-ga
In the file book.json (which you may need to create) --
You can set the Google Analytics tracking ID using the plugins configuration in the book.json:
{
	"plugins": ["ga"],
	"pluginsConfig": {
		"ga": {
			"token": "UA-XXXX-Y"
		}
	}
}
There is also traffic reporting at gitbook (assuming you've published your book that way)