Sundanese language
Coolsma continued to write about both missionary work and the Sundanese language. In 1881, he wrote a series of condemnatory reviews of Sundanese-language schoolbooks offered by the colonial government, arguing that the content had little value and the language was mostly artificial. He published a Sundanese-Dutch dictionary in 1884, consulting various works of Sundanese literature for his lexemes. In 1901, he published a history of the mission in the East Indies, titled "De zendingseeuw voor Nederlandsche Oost-Indiës" ("The Century of the Mission in the Dutch East Indies"). It described the 19th century as time of extensive growth.