Many of these pages seem to be scans. There's a 1531 date on the Kalendar; a note in the Introduction says that:
MUSIC OF THE SARUM OFFICE takes as its basis the text of the Breviarium ad usum Sarum of 1531. Reference is made to the edition of 1879-1886 for clarification and for explanatory notes. The publication of this work commences with the Psalter, the chants of the office for each day of the week followed by the Common of the Saints. This is followed by the Temporale, the chants for the Kalendar of the year and the Sanctorale, or Proper of the Saints. Folio numbers of the original edition appear in the margin. To this text is added the music found in the Antiphonale Sarisburiense (1901-1924), the sources of which stem from the early 13th century. Musical items not available here are made good from other sources such as the Processionale (1502),the Antiphonale (1519-1520) and the Hymnale (1525). The resulting work presented here is thus a Noted Breviary that represents the Use of Sarum in the late 15th and early 16th centuries.
At present the rubrics appear only in the original latin. Readers may feel the need for English translations of the rubrics. Readers are encouraged to contact the editor regarding errors or omissions, as well as for clarification of matters of style and performance. The web-based publication of this document is intended to facilitate ongoing attention to these issues.
Hmmmm. Being part of the translation into English would be fun, wouldn't it?
Well, there it is - the beginning, anyway.