Wikipedia's article is not a bad place to start.
Action learning began, for me, as practitioner led engaged, participatory research and has become important for leadership and management of innovation and change. Action learning has long been used by teachers in classrooms to identify need for change, devise and implement changes to methods, materials, curriculum, and monitor and evaluate effects of change. Action lerning produces rich, detailled, local accounts of learning and development.
Learning Thru Action and Action Learning International are sources of resources on action learning, largely in leadership contexts.
Marquardt, M.J., Leonard, S., Freedman, A., and Hill,C. 2009. Action learning for developing leaders and organizations. Washington, DC: American Psychological Press