The ISO 20022 standard is described in the document "ISO 20022 Financial Services - Universal financial industry message scheme".
The current edition of the standard includes eight parts, published in May 2013:
Orders for ISO 20022 and other International Standards or ISO publications can be obtained through www.iso.org. In the Search box, type in 20022, then Search. The eight parts of the standard are available for purchase in either a PDF or paper version.