While GeoMason is still undergoing development. Mark Coletti has put together a Cookbook for 'recipes' for using GeoMason. The Cookbook is available from here.
It gives examples of how to read and write geospatial data along with using geospatial data within your agent-based model (e.g. having an agent follow a gradient) and much more. Each example is associated with a GeoMason model example.
Mark welcomes any comments so please send them. For example, what GIS functions do you want more information on? What are the common problems you have encountered using GeoMason?