So... why this page?
It started with a list of Gregorian books, therefore the name. Cause I wanted to understand a bit more about the various books you can find on Gregorian chant, especially the Solesmes ones. Dating them in history was important as it tells the moment in time they were used for, which liturgy they were serving.
And bang! I realized liturgy was important too and to be linked to Gregorian books. Liturgy is the context in which Gregorian books are used. Dom Guéranger remains famous for the restoration of Solesmes monastery in XIX century and the promotion of Gregorian chant, but his main work is on the Roman Catholic liturgy.
So I added lines on liturgy. And when one thinks liturgy, one thinks Vatican, where the rules come from, especially Vatican II. So I added rows for encyclicals, motu proprio, etc. Eventually I completed the table with the Popes, because they are the ones who guide the Church, aren't they?
I also added some rows on history, history of the Church/monasteries, history of France/England.