https://en.wikipedia.org/wiki/Secret_Belgian_binding
https://archive.ph/20140407123450/http://users.stargate.net/~dearsam/bookarts/
https://myhandboundbooks.blogspot.com/2011/06/secret-belgian-binding-not-secret.html
https://www.ibookbinding.com/blog/top-10-secret-belgian-binding-tutorials/