If you want to convert one or more HTML files to EPUB quickly using the command line you can try pandoc.

pandoc -f html -t epub3 -o output.epub input.html

You can add metadata for epub:

pandoc -f html -t epub3 --epub-metadata=metadata.xml -o output.epub input.html

You can find other resources here:

How to convert various eBook formats for Amazon Kindle on Linux