calibre

calibre is what you install the first time a book arrives in a format your reader refuses to open. It converts between every ebook format that matters, keeps a library with proper metadata and covers it fetches for you, edits EPUB files directly when a publisher's file is broken, and pushes books to a Kindle or Kobo over USB or by email. There is also a content server, so your library becomes a web page on your own network that any phone in the house can read from.

Kovid Goyal has been writing it since 2006 and still ships releases most weeks, which is an unusual thing to be able to say about free software. Nothing is gated, there is no account, and the donation on the download page is genuinely optional.

Two honest limits. The interface is a wall of toolbars and it is not going to charm you — the app is organised around capability, not taste. And current builds need macOS 14 or newer, so on an older Mac you are pinned to whichever release last supported it.

If you own three EPUBs and a Kindle, Amazon's own Send to Kindle page does the job and you do not need this. If you have a few hundred books, or you care what format they are in ten years from now, calibre is the tool that keeps them yours.