mdmerge not working as expected

[pkriens]

The mdmerge tools seems very interesting but I can't get it to work. When I use it, it just copy the input to the output.

(master *)$ mdmerge --export-target html -o t.html book.md
(master *)$ more t.html 
<<[frontmatter.md]
<<[foreword.md]
<<[introduction.md]
<<[atoms.md]
<<[expressions.md]
<<[relations.md]
<<[fields.md]
<<[puzzles.md]

<footer>
      This is the text that goes at the bottom of every page.
</footer>
   (master *)$ mdmerge --version
   mdmerge version 1.0.1
   Copyright 2015 Dave Hein. Licensed under MPL-2.0.

Any ideas?