The best open source alternative to Amazon Kindle is calibre. If that doesn't suit you, our users have ranked more than 50 alternatives to Amazon Kindle and 19 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Amazon Kindle are Document Viewer (Free, Open Source), Foliate (Free, Open Source), Librera Reader (Freemium, Open Source) and Cool Reader (Free, Open Source).
Calibre is a free and open source e-book library management application made by users, for users. Many languages, designed around the concept of the logical book, i.e., a single entry in your library that may correspond to actual e-book files in several formats.
Most users think calibre is a great alternative to Amazon Kindle.
KOReader is a document viewer for E Ink devices. Supported fileformats include EPUB, PDF, DjVu, XPS, CBT, CBZ, FB2, PDB, TXT, HTML, RTF, CHM, DOC, MOBI and ZIP files. It’s available for Kindle, Kobo, PocketBook, Android and desktop Linux.