Litho is a page layout and publishing app. It is an open-source alternative to a professional layout app, built from scratch by NC. If you know a professional layout app's tools, menus and panels, you will find them here.
It has spreads and parent pages, text frames that thread from one to the next, paragraph and character styles, swatches and text wrap. It opens and saves a professional layout app Markup (IDML) files and exports PDF, PNG and EPUB. It is free, with no subscription, licence server or telemetry. The project is in pre-alpha, so expect gaps.