He explains, “We are called continually to rediscover the richness of the general principles exposed in the first numbers of Sacrosanctum Concilium, grasping the intimate bond between this first of the Council’s constitutions and all the others.”
Sacrosanctum Concilium was the first document promulgated by the Second Vatican Council and laid the groundwork for the reform that was to be initiated after the Council ended.
However, it is a document that is often under appreciated and insufficiently read by the lay faithful.
To help correct that trend, Pope Francis references the document 14 times in his short letter.
Pope Francis even notes how often he refers to the document when reflecting on the art of celebration, “One way of caring for and growing in a vital understanding of the symbols of the Liturgy is certainly the ars celebrandi, the art of celebrating. This expression also is subject to different interpretations. Its sense becomes clear if we refer to the theological sense of the Liturgy described in Sacrosanctum Concilium n. 7, to which I have already referred several times.“
In summary, if you want to fully understand the reform of the liturgy and the desires of Pope Francis and his last few predecessors, read Sacrosanctum Concilium.