Free Ink is an open source project for e-reader OS

Free Ink is an open-source collective building the software, firmware, and hardware for e-paper readers.. Community-built, fully open-source firmware for budget e-paper readers, with more features, more control, and an open base anyone can build on. It currently supports Xteink X4, Xteink X3, de-link, LilyGo T5 S3, Sticky, and M5Paper v1.1.

Free Ink parses EPUB 2 and 3, applies embedded CSS, and lays out chapters in your font, size, and margins. Pages are cached to SD on first open, so every reopen is near-instant. Noto Serif and Noto Sans built in, plus any font loaded from your SD card. Tune size, spacing, margins, hyphenation, alignment, and anti-aliasing. The reader runs an upload server over WiFi. Drop EPUBs in from any browser, send straight from Calibre, and keep your place across devices with KOReader sync. Full right-to-left layout for Hebrew and Arabic, plus translated menus in Spanish, French, German, Italian, Portuguese, Russian, Ukrainian, Polish and more.

Not only is several e-readers fully supported, but the company has a software framework on GitHub that allows developers, hackers, tinkers and brands to experiment on their own hardware.