The ISO 20022 standard is described in the document "ISO 20022 Financial Services - Universal financial industry message scheme".
It includes five parts:
A sixth part, ISO 20022-6 Message Transport Characteristics, was published in 2009, but has not yet been implemented.
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 six parts of the standard are available for purchase in either a PDF or paper version.