Tellico
Tellico is a free and open source software for organizing various collections
GCstar
GCstar is a free open source multi-platform software for managing your collections
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