You can buy the complete standard specifications at the ISO web page, or translated (in some countries) through your national standardization organization. You can also check your University or Organization library to know if you have free access to the specifications. Additionally, you can read the ISO 13606-1 introduction section freely.
ISO 13606 is composed of five parts. Parts 1, 2 and 3 are the most relevant for a basic implementation of the standard.
Download the latest version of the ISO 13606:2008 XML Schema.
Download the latest version of the ISO 13606:2018 XML Schema.
Reference archetypes defined in ISO 13606 Part 3 will be available soon.