Custom domains are now enabled for private and paid books.
Public books and homepages are served under
https://username.gitbooks.io/book/. We’re now enabling these domains for private and paid books, allowing for a more reliable and secure reading experience.
Private and paid books can also link a custom domain to serve content. Add your custom domain name in your book settings, then point your DNS to GitBook.
HTTPS isn’t enabled for books using custom domains, this isn’t yet possible for technical reasons. That might change in a few months thanks to the Linux Foundation’s Let’s Encrypt effort.