Pachinko by Min Jin Lee — a multi-generational saga beginning in early 20th-century Korea, following a Korean family through Japanese colonial rule and into post-war Japan. Dense, moving, and probably the best single novel for understanding the experience of Koreans in the 20th century. Hard to put down.
Human Acts by Han Kang — Han Kang's most political novel, set during and after the 1980 Gwangju Uprising, when the army was sent in to suppress pro-democracy demonstrations with brutal results. A deeply unsettling book about violence and the persistence of memory. Essential context for understanding modern Korea.
The Vegetarian by Han Kang — Han Kang's most celebrated work internationally; a short, disturbing novel about a woman who stops eating meat and what that decision triggers in the people around her. Less historical than her other books, more interested in the psychology of conformity and control.
The Impossible Country by Daniel Tudor — a non-fiction account of modern South Korea by a journalist who spent several years in the country. Covers the economy, social pressures, drinking culture, religion, North Korea relations, and the K-pop industry. The most readable general overview of contemporary Korean society available in English.
Korea: The Impossible Country covers much the same ground but is more analytical; Tudor's updated editions deal with the post-2010 Korean Wave.
The Island of Sea Women by Lisa See — historical fiction set on Jeju Island, following two haenyeo (female divers) from the 1930s through the Jeju April Third Incident of 1948, when the island was devastated by military suppression of a leftist uprising. Good on the specific Jeju culture, and a massacre that remains little known outside Korea.
Matterhorn by Karl Marlantes — not set in Korea but gives the most vivid sense of what the Korean War meant for American soldiers; better read alongside Don Oberdorfer's The Two Koreas, a solid history of the peninsula's division up to the 1990s.