Hi @zhuok: I don't think there is a single doc which speaks about complete CS. Below are some ref which can help to understand about PEGA customer service.

About and features: https://www.pega.com/products/customer-service 

Links for training docs, and other resources: https://community.pega.com/products/customer-service

Documentation: https://docs.pega.com/bundle/customer-service-88/page/customer-service/hub/customer-service-getting-started.html

Academy mission for developer: https://academy.pega.com/mission/pega-customer-service-implementation/v1

Academy mission for foundation: https://academy.pega.com/mission/pega-customer-service-foundation/v5

Latest update details: https://academy.pega.com/mission/pega-customer-service-release-update/v4

Thanks.