Open Source Ereader
An ebook library is a collection you assemble over years, and the quiet risk is that the reading app and the store are the same company - so a license can be revoked, a format can be locked, and a book you paid for can vanish from a device you own. The open source readers and library servers here keep your books as open files you manage yourself, converting between formats and serving your own shelves over the network, so your library answers to you rather than to whoever sold you the last chapter.
KOReader
Document viewer for e-ink readers with broad format support and device-specific installs
calibre
E-book manager for viewing, converting, editing, cataloging, and sending books to reader devices
Readest
Open-source ebook reader for desktop, mobile, and web with notes, TTS, and cross-platform sync
Calibre-Web
Web app for browsing, reading, and downloading eBooks from a Calibre database
Kavita
Self-hosted reading server for comics, manga, EPUB, and PDF with built-in readers and OPDS
Foliate
E-book reader for Linux with EPUB, MOBI, AZW3, FB2, and comic archive support
Calibre-Web Automated
Self-hosted digital library software that automates eBook ingest, conversion, and organization
Thorium Reader
Cross-platform EPUB reading app with OPDS import, layout controls, and accessibility support
Vivliostyle
Open source CSS typesetting engine for HTML, EPUB, and web publications