The focus of this article are the letters in Thomas Mann’s Novel Buddenbrooks. While the intersection between the letter and the novel is widely explored in the case of the epistolary novel, lesser attention has been given to letters which are simply inserted in the novels. In the attempt of filling this theoretical gap, the article faces the question if the epistolary inserts in 19th Century German Novel could be seen in relationship with the the epistolary novel. This leads to a general analysis of the function of letters in fictional texts and to a comparison between 18th and 19th epistolary culture. After this short introduction, the article focuses on the discussion of ten letters which we can find in Thomas Mann’s novel. Their function is not merely to evoke distant spaces, in which distant characters live, but also to outline the intimate relationship between the member of the family. Jean Buddenbrook uses letters to his children to give them general advices about their duties towards the family ethics. Two sections, in which we find three interconnected letters, present themselves as little “epistolary novels” inserted in the wider plot of the novel. After the end of its classical era in the late 18th Century, we may then consider Buddenbrooks as an important step in a larger development that leads to the rediscovery of the epistolary form of writing in early 20th Century.