Balhaego is a history book about the Balhae

Balhae

Balhae was a Manchurian kingdom established after the fall of Goguryeo. After Goguryeo's capital and southern territories fell to Unified Silla, Dae Jo-yeong, a Mohe general, whose father was Dae Jung-sang, established Jin , later called Balhae.Balhae occupied southern parts of Manchuria and...

 civilization, written by Yu Deuk-gong

Yu Deuk-gong

Yu Deuk-gong was a Korean scholar during the Joseon Dynasty. He is remembered today for his work in recovering the history of Balhae, which had not generally been considered part of Korean history before his time.-See also:*History of Korea...

 who was a Silhak

Silhak

Silhak was a Korean Confucian social reform movement in late Joseon Dynasty. Sil means "actual" or "practical," and hak means "studies" or "learning." It developed in response to the increasingly metaphysical nature of Neo-Confucianism that seemed disconnected from the rapid agricultural,...

 (practical studies) scholar, during the Joseon Dynasty. It describes the king of Balhae, the vassals of Balhae, geography and geology, government system, foods and clothing, etc. It is an early book that argued that Balhae should be included as part of Korean history.