Below is a list of readings that I have found helpful in explaining some part(s) of Korean philosophy. I hope you can find them useful as well.
Comprehensive overviews:
Sourcebook of Korean Civilization, 1993 - Edited by Peter Lee
Korea - A Religious History, 2002 - James Grayson
Korean Philosophy: Sources and Interpretations, 2015 - Edited by Youn Sa-soon
Religious and Philosophical Traditions of Korea, 2019 - Kevin Cawley
https://plato.stanford.edu/entries/korean-philosophy/ - Halla Kim
Korean Confucianism:
The Rise of Neo-Confucianism in Korea, 1987 - Wm. Theodore de Bary, JaHyun Kim Haboush
To Become a Sage: The Ten Diagrams on Sage Learning, 1988 - Michael Kalton
https://plato.stanford.edu/entries/korean-confucianism/ - Kevin Cawley
Korean Buddhism:
The Formation of Ch'an Ideology in China and Korea, 1989 - Robert E. Buswell
Makers of Modern Korean Buddhism, 2010 - Jin Y. Park
https://plato.stanford.edu/entries/korean-buddhism/ - Jee, Lucy Hyekyung
Korean Daoism:
Daoism in Korea (book chapter) In: Daoism Handbook, 2000 - Jung Jae-Seo
Taoism in Korea (entry) In The Encyclopedia of Taoism, 2004 - Fabrizio Pregadio
Taoism in Korea- Past and Present, 2010 - Donald Baker
Toegye’s Appraisal of Daoism, 2020 - Vladimir Glomb
Korean Shamanism:
Concerning the Origin and Formation of Korean Shamanism, 1973 - Jung Young Lee
Korean Shamanism: The Cultural Paradox, 2003 - Chongho Kim
The Concept of “Korean Religion” and Religious Studies in Korea, 2010 - Chongsuh Kim
Original Texts:
Online Courses: