This article focuses on the concept of "letter" in oral folklore. The main research material is examples from the older folk songs of Seto, where a letter, a book and other items referring to literacy are mentioned. Texts under consideration are poetical and the meaning conveyed in them is not always very clear. The term "letter" may be related to a message, paper, book, leaf or other material medium in the songs. The boundary between oracy and literacy is therefore thin, given that letters and writing are often imagined and conveyed in a physical context. Literacy in Seto folk songs is sometimes reflected as part of mythological knowledge and a mythological worldview. The written text or objects carrying it may have magical power. In some songs the writing can also be found on plants, which is interesting from a cross-cultural perspective. Similar motives from the folklore and beliefs of other peoples have been used comparatively to understand the content of the songs under consideration. The song of heavenly cows eating holy plants offers an opportunity to draw intercultural parallels and raises the question about the ethnogenesis of the Seto and their relatedness to different Eurasian peoples further to the east.