Today's scholas working within the ordinary form have a great appreciation for The Gregorian Missal, which provides most all the music they need for Sundays and Feasts throughout the year.
Before The Gregorian Missal there was the Liber Brevior, which I believe came out in the 1950s. It shortened the vast resources available in the Liber Usualis to just what the schola needed to sing at Mass. So 2000 pages became 800 pages, and it is a very nice size print. I'm not sure how widely circulated the book was at the time, but it is pretty clear that the Gregorian Missal is based on this model.
The good people at Preserving Christian Publications have now reprinted the Liber Brevior, and this is a wonderful thing. It is a fraction of the price of the Liber Usualis, and mostly meets all the needs of a schola that sings for the extraordinary form. It includes only Masses for Sundays, so you avoid the bulk and complexities of the full Graduale and you don't have to sort through all the extras you find in the Liber Usualis.
This is a very helpful and nice resource, and the price is really right. The editions of music of course are stable across all of these books - all of them prepared by the monks of Solesmes. This makes singing at Mass a straight-forward matter.
And if you are good at calendar conversions, you could use this at the ordinary form too.
And congratulations to PCP for doing all of this. There can be much money in this work, and yet their prices are reasonable and their quality first rate.