Imbens and Rubin’s book “Causal Inference for Statistics, Social, and Biomedical Sciences: An Introduction” is an excellent reference for the Potential Outcomes approach. There are also several summaries online done by Rubin which do a great job of explaining the core concepts and how they apply in concrete examples. Rubin’s class in grad school was a large factor in my decision to focus my PhD on Causal Inference, and that book is one I return to frequently.