Buka
Buka is a book manager with a simple and minimalistic user interface for organizing and reading PDF books
csBooks
csBooks – manager and reader of eBooks in PDF, EPUB and MOBI formats
Bookworm
Bookworm – free and open source software simple, focused eBook reader for Linux
Polar
Polar – free and open source cross-platform software for organizing and reading PDF files, capture web content, with support for adding annotations
Foliate
Foliate – free and open source simple and modern eBook viewer for Linux desktop
Eplee
Eplee is an ePub reader focused on reading without distraction with a simple and beautiful user interface (UI)
Lucidor
Lucidor – free and open source multitype software for reading and handling e-books
Peruse
Peruse is a free multi-platform open source software for reading books and comics from KDE
KOReader
KOReader an ebook reader application supporting PDF, DjVu, EPUB, FB2 and many more formats, running on Cervantes, Kindle, Kobo, PocketBook and Android devices
Calibre
Calibre is a free, open source cross-platform software for reading, converting, and organizing e-book collections