So after reading this, I simply typed in "catholic natural law" into Amazon to see what books would pop up.
Searching for a Universal Ethic: Multidisciplinary, Ecumenical, and Interfaith Responses to the Catholic Natural Law Tradition https://www.amazon.com/dp/0802868444/ref=cm_sw_r_awd_-Qubvb0YCRD8S https://www.amazon.com/dp/0802868444/ref=cm_sw_r_awd_-Qubvb0YCRD8S
This was the first listing. It seems like an interesting read, might get you started in the right direction. If anything, ask your priest! They're very well-read individuals. Good luck!