Art Direction:
Infamous Second Son Art Direction
Dota 2 Character art design guide from Valve
Autodesk Art Guides
GDD Samples:
Drexel GDD Resource Files
Other GDDs
anatomy_of_a_design_document
BaldwinGameDesignDocumentTemplate.doc
Gamasutra Play_With_Fire GDD
MS Word video-game-design-document-template
Epic Defense Production Journal