You probably already know this, but as a reminder:

• sed is for inserting, replacing and deleting. -i causes sed to edit in place.

• pandoc is for converting between text formats like HTML, Markdown, RST, TeX, various wikis…

• calibre has command-line subtools like ebook-convert which can produce good EPUB from lots of other formats.

• libreoffice handles typical office document tasks. Graphical.

• sigil is specialized as an EPUB authoring tool. Graphical.

• scribus can do page-layout tasks up to full daily newspapers. Graphical.

With those and a good text editor, you can produce good-looking documents ranging from a business letter through a book.