Details
The best overview of ArchiMate is given by the book:

Marc Lankhorst et al., Enterprise Architecture at Work - Modelling, Communication and Analysis, Berlin: Springer-Verlag, 2005.
If you want to know the precise details of the ArchiMate language, e.g. to implement it in a tool, the following two documents are important. They give a precise description of all concepts and relations, the backgrounds of the metamodel and other details:
Henk Jonkers (ed.), Frank de Boer, Marcello Bonsangue, René van Buuren, Luuk Groenewegen, Stijn Hoppenbrouwers, Marc Lankhorst, Erik Proper, Andries Stam, Leon van der Torre & Gert Veldhuijzen van Zanten, Concepts for Architectural Description, Enschede: Telematica Instituut, 2003-2004.
René van Buuren (ed.), Stijn Hoppenbrouwers, Henk Jonkers, Marc Lankhorst & Gert Veldhuijzen van Zanten, Architecture Language Reference Manual, Enschede: Telematica Instituut, 2003-2005.
Much more background material can be found here.