Article on the 1911-1913 revamping of the breviary so as to allow recitation of all of the Psalter each week. Feasts were also ranked according to liturgical importance, and some offices were no longer obligatory or were even suppressed.
http://www.newadvent.org/cathen/16013a.htm
Published by the Congregation for Divine Worship in 1971.
http://www.catholicliturgy.com/index.cfm/FuseAction/DocumentContents/Index/2/SubIndex/39/DocumentIndex/2
By William J. Lallou. Written in 1950, so the four books he mentions do not correspond to the four-volume set in use today.
http://www.ewtn.com/library/PRAYER/DIOFFICE.TXT