Engestrom, Y. (1990). Learning, working, and imagining: Twelve studies in activity theory. Helsinki, Finland: Orienta-Konsultit Oy.
I am currently using it to describe the CatchBob activity.

Engestrom (1990) proposes a schematic view of activity involving mediation. &&In the model, the subject refers to the individual or subgroup whose agency is chosen as the point of view in the analysis. The object refers to the &raw material' or &problem space' at which the activity is directed, and which is moulded or transformed into outcomes with the help of physical and symbolic, external and internal tools (mediating instruments and signs). The community is composed of multiple individuals and/or subgroups who share the same general objective. The division of labour refers to both the horizontal division of tasks between the members of the community and to the vertical division of power and status. Finally, the rules refer to the explicit and implicit regulations, norms and conventions that constrain actions and interactions within the activity system.''