One of the most common mistakes that prevents EPUB validation with a retailer is a mismatched ISBN. This means your 13-digit eISBN does not match your eISBN in your OPF file. Luckily, this is an extremely easy fix to make.
Step One – the ONIX file
In ONIX the <ProductIdentifier> tag is repeated up to three times if you have an ISBN-10. Since you will only have a 13 most of the time for ebooks, you will have two <ProductIDTypes>, 03 and 15. They both need to be the same.
Step Two: the OPF file in your EPUB
Next, open your OPF file. The first few lines will look like this:
<?xml version="1.0"?><package xmlns="http://www.idpf.org/2007/opf" version="2.0" unique- identifier="9781926743035">
The above unique-identifier attribute is the first instance you’ll need to check. (In your OPF file, this will usually be displayed on one line.) This attribute is used to signify the primary identifier for your book. It tells the retailer, distributor, whatever software that reads to link this EPUB with the ISBN.
In this example, we used the ISBN to name the unique identifier (indicated in the bold text) for simplicity. You can input any text here – common variations are “bookid” and “p9781926743035” (p followed by the ISBN). No matter what you call your unique id, the important part is that you use the same text in the bold portion of the code below.
Next comes the metadata. There may be ten lines or so, but for ISBN purposes we’ll concentrate on one:
The identifier element (that’s the text between the two tags) is always your ISBN-13.
With these simple fixes, you can get your EPUB on its way into retail stores, and into the hands of your readers.