During her seclusion preceding her marriage (1891-1903), Kartini wrote a large number of letters to (mostly) Dutch friends. The majority of the letters (inventory no. D H 1200) were addressed to Rosa Manuela Abendanon-Mandri, the wife of a civil servant in the Dutch administration. Their grandson donated the letters to the Royal Netherlands Institute of Southeast Asian and Caribbean Studies (KITLV). Smaller groups of letters were added later through donations by the historian C. Fasseur (2001) and the Van Vollenhove Institute for Law, Governance and Society (VVI) (2014). More letters and (parts of) Kartini’s archive can be found in the National Archives in The Hague and the Arsip Nasional Republik Indonesia (ANRI).

In her letters, Kartini raised questions about the position of women in traditional Javanese society. Arranged marriages and polygamy did not have her approval.

Through personal experience she was familiar with the consequences of these customs. Even worse was the exclusion from education. This was partly the result from the seclusion of women prior to marriage – going to schools was prohibited – but also from the opinion that women didn’t need education to fulfill their role in society. To Kartini, education had been an essential instrument of personal development and she was lucky to be allowed the use of books, while living at home. Before entering her marriage, it was stipulated that Kartini would be allowed to continue her studies (in private), and to found a school for girls.

Kartini's letters were first published in 1912, under the title 'Door Duisternis Tot Licht'. In 1922 they were translated in Malay and published as 'Habis gelap terbitlah terang'. Many editions followed in many different languages. In 2014 Joost Coté published Kartini's 'Complete Writings 1898-1904'. The letters still appeal to a modern audience. In 2025 a new (Dutch) anthology was published: 'De mooiste brieven van Kartini'.