I you haven't already seen this at New Liturgical Movement or on Dominican Liturgy Blogspot, Fr. Augustine Thompson, O.P. has posted a link to a full color, downloadable copy of the prototype Dominican liturgical books that were produced under the direction of Master General Humbert de Romans. As Fr. Thompson explains it:
"This manuscript was compiled by the Master of the Order Humbert of Romans in accord with the commission of the Dominican General Chapter of Buda in 1254 and approved by the General Chapter of Paris in 1256. Except for additional feasts and the Psalter Reform of St. Pius X, the texts of this document were authoritative as the prototype for our liturgical books. This copy was carried by masters of the Order during visitations to correct the books of our houses.
Those who would like copies of the images for each of the parts may download them as zip files on the left sidebar under the heading "Dominican Chant Books--Downloadable." I have divided the files so that they are not excessively large (they total 2.8 gigabytes). Even, so some of these files, especially those with lots of music, like the Graduale and the Antiphonarium are very large. So do be patient as they download.
Thank you Fr. Thompson!
Before the listing of the files, there is also a link to download the typescript index prepared by Fr. William Bonniwell in the 1950s. It will make it much easier to find particular texts or music."