In their text files, Project Gutenberg uses -- as an em dash. The Book Cleaner files
https://www.mobileread.com/forums/showthread.php?t=11649 for Book Designer do make sure em dashes do not get converted to a dash and that -- gets converted to a proper em dash.
Whoever, if you do start with a PG text file, load it into Word and convert it to RTF as Book Designer can mess up paragraphs in text files on load. RTF and HTML load fine.
- dash
– en dash
— em dash