A little odd, but...
I happened to need to copy over my calibre libraries from a different user account tonight, but was stopped due to missing read permission. The odd thing was, this
only applied to books that were touched by ebook-edit. Converted books, added books, opfs, db and prefs backups all had permissions of 664, but a list of books which happens to be perfectly identical to "any book I have ever opened in the editor" has permissions of 600.
I cannot say this is the worst thing ever to happen, in fact I doubt whether it will ever be an issue again (I'm surprised it came up to begin with
) but I thought I'd mention it.