The official specification of the DITA architecture is available from OASIS.

OASIS DITA Version 1.1. Architectural Specification

The architectural reference defines both a) a set of document types for authoring and organizing topic-oriented information; and b) a set of mechanisms for combining and extending document types using a process called specialization.

The specification consists of:

• The DTDs and schemas that define DITA markup for the base DITA document types, as well as catalog files. While the DTDs and schemas should define the same DITA elements, the DTDs are normative if there is ever any discrepancy.
• The language reference that provides explanations for each element in the base DITA document types
• This document, which has the following parts:
  
  • an introduction, which provides background concepts and an overview of the architecture
  • the DITA markup specification, which provides an overview of DITA's base document types
  • the DITA processing specification, which provides descriptions of common DITA processing expectations
  • the DITA specialization specification, which provides details of the mechanisms DITA provides for defining and extending DITA document types.

The architectural specification is available online as a set of HTML files, and can be downloaded as a set of DITA XML files. The DITA files have been compiled into an Eclipse Infocenter help system that is online at DITA Infocenter.  The Infocenter is indexed, has a table of contents, and can be full-text searched.

The architectural specification has been converted to MediaWiki format on the DITA Wiki. There you can add comments to the specification.