The literature of the nonprofit sector has ample materials related to volunteerism and volunteer engagement. While this is not meant to serve as an exhaustive list, the following organizations offer additional research and information.
Center for Nonprofit Excellence
The Leighty Foundation
Grantmakers for Effective Organizations
Points of Light
America’s Service Commissions
Rosenthal, Robert J. ed., Volunteer Engagement 2.0: Ideas and insights changing the world. John Wiley & Sons. 2015.
Stallings, Betty with Susan J. Ellis. Leading the Way to Successful Volunteer Involvement: Practical Tools for Busy Executives.
Stallings, Betty. 55-Minute Training Series