The Sarum gradual and the Gregorian antiphonale missarum: a dissertation and an historical index
By Walter Howard Frere, Plainsong and Mediaeval Music Society (Great Britain), Catholic Church, Plainsong and Mediaeval Music Society (Great Britain).
Published by published for members of the Plainsong and Mediaeval Music Society [by] B. Quaritch, 1895
Original from Harvard University
Digitized Oct 29, 2007
You can download a 3M PDF of the book, too (that's the link right there).
Haven't looked very closely at this yet, but I'm sure it will be very interesting. There are some (black and white) samples of illuminated chant manuscripts, too, such as the one below, which I believe is Plate 1, and is described this way:
1. From Brit. Mus. MS. Additional, 12194—tne MS. which is the main basis of the reproduction given in the complete work. Sec p. xxxv.
This page contains the end of the Service for Saturday in the September Ember Week, and the beginning of the Procession on the Dedication Festival.