All books on Gitbook.io are accessible via the url http://{{author}}.gitbooks.io/{book}/, and content is accessible at http://{{author}}.gitbooks.io/{book}/content/.
But you can configure your book to use a custom domain name (a free feature on GitBook). Domain name can be use for the homepage and the content (or both).
The process to add a custom domain to your book is easy.
In order to use your own domain, you will need to make a few changes with your domain registrar:
Log in to your domain registrar and find the section that allows you to add/edit host records, often found in a settings menu called 'Edit DNS', 'Host Records' or 'Zone File Control'.
Set the www record to a CNAME and set the URL field to: www.gitbook.com.
To redirect the naked domain (yourdomain.com) to www.yourdomain.com, find the option to forward your domain. This can usually be found under 'Forwarding', 'URL Forwarding' or 'URL Redirect'.
It may take a few hours for domain changes to propagate.