Duchess Charlotte dedicated her famous journal, written between 1775 and 1817, to her friend Sophie Piper, and until 1800, the journal is formally written in the form of letters to her. The letters were never sent, but written in this way as a dedication to Sophie Piper. The diary has been published between 1902 and 1942, and regarded as a valuable reference work of contemporary Sweden. The two friends also upheld a vast correspondence, which has been partially published. Duchess Charlotte once referred to Sophie Piper as the only true friend she ever owned, and upon the death of Piper in 1816, Charlotte wrote a biography of her.