The official site:

http://www.bay12games.com/dwarves/ (it is free btw, they lives off donations)

 

Best outsider article I have read to date:

http://www.nytimes.com/2011/07/24/magazine/the-brilliance-of-dwarf-fortress.html?_r=2&ref=magazine&

 

A good game developer-centric article:

http://www.gamasutra.com/view/feature/131954/interview_the_making_of_dwarf_.php

 

There is no manual really... just a lot of discovery... however you choose in game or out.

http://dwarffortresswiki.org/index.php/Main_Page

 

Here is the bit about inclusion in MOMA

http://www.moma.org/explore/inside_out/2012/11/29/video-games-14-in-the-collection-for-starters/

 

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