Unlike Dropbox, Proton Drive interacts poorly with Calibre.

Initially, I thought I had successfully migrated my Calibre Library from Dropbox to Proton Drive. Thinking everything was okay, I even deleted my Dropbox account. But just a few days later, disaster. 100% of my books are gone. Ten gigabytes of thousands of carefully curated ePUB files. All that was left were empty, useless author folders.

Although Proton Drive claims to support recovering old files within 180 days by default, the option to restore my Calibre Library folder from a previous version failed to appear in the menu.

If you value your ebook collection, do NOT use Calibre. Use plain folders, organized however you like.

Use some combination of USB and/or Amazon Whispersync sideloading. If you require tweaking individual books, use Sigil.

When in doubt, purchase paper editions of books which include DRM-free downloads.

Proton should improve folder version recovery and improve desktop syncing to integrate better with Calibre. Personally, I've lost all trust in Calibre and would not recommend that fragile application in any context.