Hi berrybatens
The ISBN field is really zencart product 'model' field, so it's not one that can be easily duplicated and then appear exactly the same as ISBN does, (e.g ISBN appears in product listings for book products)
But the book 'misc' fields that are spare can be used as extra data lines in the product book display lower section... these are fields that take up to 255 characters.
if you have a look in the admin ....
catalog | product types | product book | edit layout
You may see that there are some fields called 'misc...' 1 to 10
Some of these are already on (1) but those set to '0' should available for use, if set to 1.
If one of those spare is switched on, (maybe avoid misc_1) it should then appear, both in admin side book product page, (collect info page), and also appears in the data lines listing on the cart book display provided a value was inserted in admin for that field.
You would have to also find the books 'defines' php file entries to change the textual labelling of the field, in both the admin (admin/includes/languages/), and the cart defines (/includes/languages/) /your_language, if you don't want labels to remain as 'misc_' which is their currectly set label text.
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