In the first book, Dante discusses the relationship between Latin and vernacular, and the search for an "illustrious" vernacular in the Italian area; the second book is an analysis of the structure of the canto or song (also spelled canzuni in Sicilian), which is a literary genre developed in the Sicilian School of poetry.
Latin essays were very popular in the Middle Ages, but Dante made some innovations in his work: firstly, the subject (writing in vernacular) was an uncommon topic in literary discussion at that time. Also significant was how Dante approached this theme; that is, he presented an argument for giving vernacular the same dignity and legitimacy Latin was typically given. Finally, Dante wrote this essay in order to analyse the origin and the philosophy of the vernacular, because, in his opinion, this language was not something static, but something that evolves and needed a historical contextualisation.
Dante interrupted his work at the fourteenth chapter of the second book, and though historians have tried to find a reason for this, it is still not known why Dante so abruptly aborted his essay. Indeed it is an unfinished project, and so information about its intended structure is limited. Though at some point, Dante mentions a fourth book in which he planned to deal with the
comic genre and the "mediocre" style, nothing at all is known about the third book. It is thought, however, that the first book was meant to be a sort of preface to the following three books, and so shorter than the others.