The Course In General Linguistics by Ferdinand de Saussure
"The Course in General Linguistics" is a foundational text in the field of linguistics that outlines the principles of structural linguistics and introduces key concepts such as the linguistic sign, the distinction between langue (language as a system) and parole (language as used in context), and the idea of synchronic versus diachronic analysis. The work emphasizes the arbitrary nature of the sign, which consists of the signifier (the form of the word or phrase) and the signified (the conceptual meaning). This book, compiled from notes by students from the lectures given by its author, has had a profound impact on the development of linguistic theory and has influenced various other disciplines, including anthropology, psychology, and literary theory.