[Grimaud]

Thank you both for your answers

Originally Posted by davidfor

None of those look to be valid ways of defining the ISBN in the OPF. At least, they are not how calibre does them.

I read them in examples from EPUB 3.2 specifications (like this one) and creating a fake book with Sigil also suggested the same way of definining it. I don’t think any of them would explain an invalid way of defining an ISBN inside an opf file.

Originally Posted by davidfor

For epub 3, it is:

<dc:identifier>isbn:9780515135282</dc:identifier>

The device read both of those as the ISBN and displays them in the details.

And the above works fro sideloaded kepubs as well.

After a small modification (the unique-identifier tag is mandatory according to epubcheck), it did work with a side loaded epub, but didn’t with the same epub ‘kepubified’, even after asking for a full scan with NickelMenu

<package version="3.0" id="package" unique-identifier="isbn" xml:lang="fr" xmlns="http://www.idpf.org/2007/opf">
  <metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
		<dc:identifier id="isbn">isbn:9791039102759</dc:identifier>

Originally Posted by PeterT

See if https://github.com/kobolabs/epub-spec helps; I do see some mention of metadata for the ISBN there.

I didn’t know about that page. I’ll try to ask them, maybe they will have a bit of time to give me an answer

Edit : I tested my first way of doing it one more time. With a side loaded .epub (something I didn't tried before), the urn:isbn:[isbn] is shown. With a kepubifyed epub, nope, nothing, what's an ISBN, etc

I tried with the exact same epub, only changing the extension from epub to kepub.epub: my Elipsa is able to read the ISBN from the .epub file, but unable from the .kepub.epub (which unlike previous tests where I used Kepubify, here, my .kepub.epub is litteraly the exact same file, except for the extension)