Calibre

Calibre is the undisputed center of ebook management: it converts between essentially all formats (EPUB, MOBI, AZW3, PDF and more), edits metadata and covers, syncs to e-readers over USB, serves your library over the network, and can even fetch news into ebook form. With DeDRM community plugins, people also use it to liberate their own purchased books — the legal status of which varies by jurisdiction, so know your local law.

The UI is famously dense and opinionated, designed by one developer over two decades — it works, but ‘pretty’ it is not, and it wants to reorganize your files into its own library structure. PDF conversion output is mediocre (inherent to PDF, not Calibre). For just reading, a lightweight reader like Foliate is more pleasant; Calibre is the librarian, not the armchair.