Service-learning essentials: Questions, answers, and lessons learned. Jacoby, B. (2015). San Francisco, CA: Jossey-Bass.
this is a good primer on service learning.
Service-Learning in higher education: Concepts and practices. Jacoby, B., & Associates (Eds.) (1996). [L220.5.S45 1996]
more detail than the 2015 version of Jacoby's work. It has a lot of definitions and criteria such as the Wingspread principles.
Learning though Serving:A student guidebook for service-learning and civic engagement across academic disciplines and cultural communities. Cress, C. M., Collier, P. J., Reitenauer, V. L., and Associates. (2013). Sterling, VA: Stylus.
a good (and inexpensive) student textbook for service learning classes
The unheard voices: Community organizations and service learning. Stoeker, R., Tryon, E. A., & Hilgendorf, A. (2009). Philadelphia: Temple University Press, 2009.
This organization developed a national program evaluation study. There is a lot of good detail on methodology and sample assessment instruments. Unfortunately their funding was cut and they were not able to actually carry out the project.