International Standards Organization (ISO) have changed the ISBN-standard from 10 to 13 digits.
All existing ISBN-10 numbers should get converted to ISBN-13 format. ISBN-10 is deprecated from January 2007.

ISBN-13 support was made in eZ publish 3.9, but with some issues. To have complete support for ISBN-13 I believe these things should be done:

• ISBN-13 should be enabled in the datatype by default on new eZ publish installations.

• If a user enters an ISBN-10 number in content-attribute that is ISBN-13-enabled, it should be converted to ISBN-13 by the datatype, and stored as ISBN-13 in the db (when ISBN-13 enabled in datatype).

• Datatype should always generate hyphens '-' at the right places. If the hyphens are stored or not in db is not so important. But it's important that the user can write ISBN-13 with or without hypens, and that the view-templates show ISBN with hyphens at correct places. Now the datatype allows hyphens anywhere, and stores, and view just as user wrote it.

• Create a script to convert ISBN-10 data to ISBN-13.

Resources for helping development:
http://www.blyberg.net/2006/04/05/php-port-of-isbn-1013-tool/
http://www.isbn.org/toolkit.html
http://www.bisg.org/isbn-13/index.html