I found two things.
For me the link has to be written '( ./6.md )' with a space before and after the file specification.
Then I found the ebook-convert app wants html files for inputs.
It complains that the inputs are "binary"
If you go to:
Preferences->Preferences->Common Options->Page Setup
you can set the Output Profile to Tablet. The Profile description reads:
Intended for generic tablet devices, does no resizing of images [Screen size: unlimited]