The Gaming Situation by Markku Eskelinen. A long investigation into what games are... "we'll use the theories of Espen Aarseth, Roger Caillois, Warren Motte and David Parlett in particular. They form a filter through which the possibly heuristic findings and borrowings from various neighbouring disciplines and predatory theory formations are viewed, tested, modified and transformed. While discussing articulation, materiality, functionality, typology and orientation, among other things, we are confronting the bare essentials of the gaming situation: the manipulation or the configuration of temporal, spatial, causal and functional relations and properties in different registers". (There's Aarseth again.)
Also. Some fantastic narrative versus story versus interaction parts, good classifications, and pulling a lot of theories together. Thought provoking portion on conventions. Highly recommended.