Internet has facilitated access to the digitalised collections of many archives and libraries around the world. However, there are still many titles, not only in Spain, that are unavailable through the web but which can be obtained in book format. This is the case of Reglas de canto plano è de contrapunto è de canto de Organo (Rules for plainchant and counterpoint and polyphony) by Fernando Esteban, which, with a first edition dating from 1375, constitutes one of the essential sources of theory for specialists in the reconstruction of theory and thinking behind Spanish music. Although this facsimile edition with its accompanying typed transcription was published over 30 years ago, today it has special value because it provides specialists with difficultly accessible material of great interest. The volume begins with a synthetical introductory study by the musicologist Mª Pilar Escudero Garcia. The organisation of the contents (facsimile on even-numbered pages and transcription on the odd numbers) makes reading them easier and allows the reader to contrast the original source with its interpretation.