It does break the border, but that's a two-line fix in ui.py to use a Rich Panel without side borders:

  +  from rich import box
     ...
     book_panel = Panel(
         book_content,
  +      box=box.SIMPLE_HEAD,
         title=f"[{COLORS.PANEL_TITLE}]{progress_text}[/{COLORS.PANEL_TITLE}]",
     ...

Then running lue in an emacs ansi-term or mlterm with a variable-width font configured, it looks like a typical ePub reader.