UN Audiovisual Library of International law includes
"The Field of Application of International Humanitarian Law: International and Non-International Armed Conflicts" by Prof Marco Sassòli University of Geneva, Switzerland
A legacy from the understanding that public international law dealt with the laws between states.
For overview & starter bibliography see article Armed Conflict, Non-International by Thilo Marauhn, Zacharie F Ntoubandi in Max Planck Encyclopedia of Public International law (link below)
Subject search terms to use in SOLO include
Guerrillas (International law)
Terrorism (International law)
Intervention (International law)