This phrase indicates that Moonstone is probably an epistolary novel, a type of genre that presents the story as a compilation of letters, diary entries, or in Moonstone 'family paper' as opposed to the direct narration of a single narrator. Here are links to information about epistolary novels from Oregon State University and Wikipedia. https://liberalarts.oregonstate.edu/wlf/what-epistolary-novel https://en.wikipedia.org/wiki/Epistolary_novel

Relating to a letter https://www.merriam-webster.com/dictionary/epistolary