Download Them from the EDitEUR Site
The most recent information about Version 3.0.8 may be located here.
As noted on the EDitEUR web page:
The ONIX family includes standards for Books, Serials and Licensing Terms & Rights Information (including RROs).
All ONIX standards are designed to support computer-to-computer communication between parties involved in creating, distributing, licensing or otherwise making available intellectual property in published form, whether physical or digital. All are expressed in XML.
ONIX for Books was the first, and is the most widely-adopted, member of EDItEUR’s ONIX family of standards. It was initially developed by EDItEUR jointly with Book Industry Communication (UK) and the Book Industry Study Group (US), and is now maintained under the guidance of an International Steering Committee including not only BIC and BISG but also national user groups in Australia, Belgium, Canada, China, Egypt, Finland, France, Germany, Italy, Japan, Korea, The Netherlands, Norway, Russia, Spain, and Sweden. The ONIX for Books Product Information Message is the international standard for representing and communicating book industry product information in electronic form.
The new codelists have been integrated into existing documentation, available from the EDItEUR website. These documents cover the latest ONIX 3.0.8: