Approval.

backend/dsb/sources.json

@@ -67,6 +67,8 @@
         "editor": "Editor",
         "editor_explica": "Editor or editors of this work. Type in and select an editor from the list. If none can be found, you can add a new one by clicking on the \"+\"-button in front of the text field.",
         "booktitle": "Book title",
-        "booktitle_explica": "The book a chapter was published in."
+        "booktitle_explica": "The book a chapter was published in.",
+        "isbn": "ISBN",
+        "isbn_explica": "ISBN of the book. Should be left empty for books published without an ISBN."
     }
 }