The repository allows for even more possibilities: automatically building the system described in a book, adding packages to a build, and adding your own book.
The same makefile that generates the book can also be used to automatically build a system. It's as easy as this:
$ git clone -b elfsMusl-20131006 git://github.com/kanj/elfs
$ cd BOOK
$ make armMusl-build
Over the next several hours a complete system will be built in the ../sysBuild/armMusl_20131006
Work in Process
Work in Process