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The Standard

If you want to know the precise details of the ArchiMate language, e.g. to implement it in a tool, the official standard is the authoritative source:
ArchiMate® 1.0 Specification, Document Number: C091, The Open Group, Feb. 2009.

This specification contains the formal definition of ArchiMate as a visual design language with adequate concepts for specifying inter-related architectures, and specific viewpoints for selected stakeholders. This is complemented by some considerations regarding language extension mechanisms, analysis, and methodological support.

Book

A good general overview of ArchiMate and its background is given by the book:

Marc Lankhorst et al. (2009), Enterprise Architecture at Work - Modelling, Communication and Analysis, second edition, Springer Berlin Heidelberg. Online version.

Much more background material can be found here.

 
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