LibreOffice is among the most feature rich and robust alternatives to Microsoft Office.

LaTeX (pronounced La-Tek) is not a word processor, but a Markup language similar to HTML.

Audacity is a cross platform FOSS audio recorder and editor that you can use to produce audiobooks.

Kid3 is an audio tagging application for tagging audiobook files with MP3 metadata required for file distribution.

Sigil is an eBook editor that you can use to convert your manuscript to a fully validated eBook.

Calibre is an eBook library manager and editor for adjusting your eBook metadata and content.

LanguageTool is an Open Source alternative to Grammarly that can check your grammar as you type.

GIMP is the most popular FOSS image editor for creating your book covers.

Krita is an open source, cross platform advanced drawing application that can create book covers.