The content of a book is placed in one directory (that directory contains
only one book, it cannot contain several books). The directory is comprised
of:
Restriction: the name of the directory the *.devhelp2 file is
in and the name of the *.devhelp2 file (minus the extension)
must match. In other words, if the book directory is called
$book_name, then the absolute path to the index file should
end with $book_name/$book_name.devhelp2. That way, when
Devhelp knows the directory name, it directly knows the location
to the index file.