Oh oh oh, I didn't specify my Q enough: I'd like to have the ISBN number in the reference list in a scanned document, I'm sorry!
And I don't know which field to use doing so, when manually typing in a reference in my database. Hope my Q makes sense now...
However, inspired by your advice, I found "References" in Preferences, and even though choosing 'book' in both columns the 'Apply' button doesn't get active - but maybe that isn't the right place to modify BE acc. to my request...
You have to choose the appropriate formatting for your output (Check out the Biblio menu, there you'll find quite a few options regarding the output formatting of your bibliography.
I do not know which formats will put the ISBN numbers into the bibliography (except for DIN 1505 - which will be in German…)
Since there are so many options, I would definitely recommmend taking a look at the manual.
Just to make sure we're all talking about the samet thing:
The ISSN or ISBN number belongs in field User6. This is usually assigned the label ISSN/ISBN. It is in the drawer -- click on the drawer icon in the upper right corner of the reference window to reveal it.