This site contains documents related to the development, use, and maintenance of the 2005 version of the Z39.86 standard. The National Information Standards Organization (NISO) provides the standard:
As a PDF file and in print for purchase.
ANSI/NISO Z39.86-2005 Specification (local copy)
2005 Version - list of most current DTDs
DTBOOK 2005 Version - html expanded version
Errata, Proposed Changes, and Maintenance Activities for Z39.86-2005