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

Uses: Writing, Editing, Interior Formatting
LaTeX Markup

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

Uses: Print book formatting

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

Uses: Audiobook Production, Podcasting
Kid3 Audio Tagger

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

Uses: Audio File Tagger
Sigil eBook editor

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

Uses: eBook Production
Calibre eBook Manager

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

Uses: eBook Management
Language Tool

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

Uses: Grammar Checker

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

Uses: Cover Design
Krita Image Editor

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

Uses: Cover Design