Here's the full entry. It's a full description of the Office as it existed, and was understood, in 1913 in the Roman Catholic Church. I'll be interested to learn if anything further has been discovered since then; I'd guess not much, but you never know. I'm also interested in finding out more about it from an Anglican perspective, and will post what I find.
I. COMPOSITION OF THE OFFICE
This office, as it now exists in the Roman Liturgy, is composed of First Vespers, Mass, Matins, and Lauds. The Vespers comprise psalms, cxiv, cxix, cxx, cxxix, cxxxvii, with the Magnificat and the preces. The Matins, composed like those of feast days, have three nocturns, each consisting of three psalms and three lessons; the Lauds, as usual, have three psalms (Ps. lxii and lxvi united are counted as one) and a canticle (that of Ezechias), the three psalms Laudate, and the Benedictus. We shall speak presently of the Mass. The office differs in important points from the other offices of the Roman Liturgy. It has not the Little Hours, the Second Vespers, or the Complin. In this respect it resembles the ancient vigils, which began at eventide (First Vespers), continued during the night (Matins), and ended at the dawn (Lauds); Mass followed and terminated the vigil of the feast. The absence of the introduction, "Deus in adjutorium", of the hymns, absolution, blessings, and of the doxology in the psalms also recall ancient times, when these additions had not yet been made. The psalms are chosen not in their serial order, as in the Sunday Office or the Roman ferial Office, but because certain verses, which serve as antiphons, seem to allude to the state of the dead. The use of some of these psalms in the funeral service is of high antiquity, as appears from passages in St. Augustine and other writers of the fourth and fifth centuries. The lessons from Job, so suitable for the Office of the Dead, were also read in very early days at funeral services. The responses, too, deserve notice, especially the response "Libera me, Domine, de viis inferni qui portas æreas confregisti et visitasti inferum et dedisti eis lumen . . . qui erant in poenis . . . advenisti redemptor noster" etc. This is one of the few texts in the Roman Liturgy alluding to Christ's descent into hell. It is also a very ancient composition (see Cabrol, "La descente du Christ aux enfers" in "Rassegna Gregor.", May and June, 1909).
The "Libera me de morte æterna", which is found more complete in the ancient manuscripts, dates also from an early period (see Cabrol in "Dict. d'archéol. et de liturgie", s. v. Absoute). Mgr Batiffol remarks that it is not of Roman origin, but it is very ancient (Hist. du brév., 148). The distinctive character of the Mass, its various epistles, its tract, its offertory in the form of a prayer, the communion (like the offertory) with versicles, according to the ancient custom, and the sequence "Dies Iræ" (q.v.; concerning its author see also BURIAL), it is impossible to dwell upon here. The omission of the Alleluia, and the kiss of peace is also characteristic of this mass. There was a time when the Alleluia was one of the chants customary at funeral services (see Dict. d'archéol. et de liturgie, s. v. Alleluia, I, 1235). Later it was looked upon exclusively as a song of joy, and was omitted on days of penance (e.g. Lent and ember week), sometimes in Advent, and at all funeral ceremonies. It is replaced to-day by a tract. A treatise of the eighth-ninth century published by Muratori (Liturg. Rom. vet., II, 391) shows that the Alleluia was then suppressed. The omission of the kiss of peace at the Mass is probably due to the fact that that ceremony preceded the distribution of the Eucharist to the faithful and was a preparation for it, so, as communion is not given at the Mass for the Dead, the kiss of peace was suppressed.
Not to speak of the variety of ceremonies of the Mozarabic, Ambrosian, or Oriental liturgies, even in countries where the Roman liturgy prevailed, there were many variations. The lessons, the responses, and other formulæ were borrowed from various sources; certain Churches included in this office the Second Vespers and Complin; in other places, instead of the lessons of our Roman Ritual, they read St. Augustine, Proverbs, Ecclesiastes, Ecclesiasticus, Osee, Isaiah, Daniel, etc. The responses varied likewise; many examples may be found in Martène and the writers cited below in the bibliography. It is fortunate that the Roman Church preserved carefully and without notable change this office, which, like that of Holy Week, has retained for us in its archaic forms the memory and the atmosphere of a very ancient liturgy. The Mozarabic Liturgy possesses a very rich funeral ritual. Dom Férotin in his "Liber Ordinum" (pp. 107 sqq.) has published a ritual (probably the oldest extant), dating back possibly to the seventh century. He has also published a large number of votive masses of the dead. For the Ambrosian Liturgy, see Magistretti, "Manuale Ambrosianum", I (Milan, 1905), 67; for the Greek Ritual, see Burial, pp. 77-8.Here's a copy in Latin - the book was published in 1722 - of the Office of the Dead (Officium Defunctorum: Sancta & Salubris est cogitatio, pro Defunctis exorare, ut a peccatis solvantur ("The Office of the Dead: Holy and wholesome to think and pray for the dead, that they may be loosed from their sins.")).
The Office of the Dead has been attributed at times to St. Isidore, to St. Augustine, to St. Ambrose, and even to Origen. There is no foundation for these assertions. In its present form, while it has some very ancient characteristics, it cannot be older than the seventh or even eighth century. Its authorship is discussed at length in the dissertation of Horatius de Turre, mentioned in the bibliography. Some writers attribute it to Amalarius, others to Alcuin (see Batiffol, "Hist. du Brév.", 181-92; and for the opposing view, Bäumer-Biron, "Hist. du Brév.", II, 37). These opinions are more probable, but are not as yet very solidly established. Amalarius speaks of the Office of the Dead, but seems to imply that it existed before his time ("De Eccles. officiis", IV, xlii, in P. L., CV, 1238). He alludes to the "Agenda Mortuorum" contained in a sacramentary, but nothing leads us to believe that he was its author. Alcuin is also known for his activity in liturgical matters, and we owe certain liturgical compositions to him; but there is no reason for considering him the author of this office (see Cabrol in "Dict. d'archéol. et de liturgie", s. v. Alcuin). In the Gregorian Antiphonary we do find a mass and an office in agenda mortuorum, but it is admitted that this part is an addition; a fortiori this applies to the Gelasian. The Maurist editors of St. Gregory are inclined to attribute their composition to Albinus and Etienne of Liège (Microl., lx). But if it is impossible to trace the office and the mass in their actual form beyond the ninth or eighth century, it is notwithstanding certain that the prayers and a service for the dead existed long before that time. We find them in the fifth, fourth, and even in the third and second century. Pseudo-Dionysius, Sts. Gregory of Nyssa, Jerome, and Augustine, Tertullian, and the inscriptions in the catacombs afford a proof of this (see Burial, III, 76; PRAYERS FOR THE DEAD; Cabrol, "La prière pour les morts" in "Rev. d'apologétique", 15 Sept., 1909, pp. 881-93).
III. PRACTICE AND OBLIGATION
The Office of the Dead was composed originally to satisfy private devotion to the dead, and at first had no official character. Even in the eleventh, twelfth, and thirteenth centuries, it was recited chiefly by the religious orders (the Cluniacs, Cistercians, Carthusians), like the Office of Our Lady (see Guyet, loc. cit., 465). Later it was prescribed for all clerics and became obligatory whenever a ferial office was celebrated. It has even been said that it was to remove the obligation of reciting it that the feasts of double and semi-double rite were multiplied, for it could be omitted on such days (Bäumer-Biron, op. cit., II, 198). The reformed Breviary of St. Pius V assigned the recitation of the Office of the Dead to the first free day in the month, the Mondays of Advent and Lent, to some vigils, and ember days. Even then it was not obligatory, for the Bull "Quod a nobis" of the same pope merely recommends it earnestly, like the Office of Our Lady and the Penitential Psalms, without imposing it as a duty (Van der Stappen, "Sacra Liturgia", I, Malines, 1898, p. 115). At the present time, it is obligatory on the clergy only on the feast of All Souls and in certain mortuary services. Some religious orders (Carthusians, Cistercians etc.) have preserved the custom of reciting it in choir on the days assigned by the Bull "Quod a nobis".
Apostolic Constitutions, VI, xxx; VIII, xl; PS.-DIONYS., De hierarch. eccl., vii, n. 2; AMALARIUS in P. L., CV, 1239 (De eccles. officiis, III, xlix; IV, xlii); DURANDUS, Rationale, VII, xxxv; BELETH, Rationale in P. L., CII, 156, 161; RAOUL DE TONGRES, De observantia canonum, prop. xx; PITTONUS, Tractatus de octavis festorum (1739), I (towards end), Brevis tract. de commem. omnium fidel. defunct.; HORATIUS A TURRE, De mortuorum officio dissertatio postuma in Collectio Calogiera, Raccolta d'opuscoli, XXVII (Venice, 1742), 409-429; GAVANTI, Thesaur. rituum, II, 175 sqq.; MARTÈNE, De antiq. ecclesioeritibus, II (1788), 366-411; THOMASSIN, De disciplina eccles., I-II, lxxxvi, 9; ZACCARIA, Bibl. ritualis, II, 417-8; IDEM, Onomasticon, I, 110, s. v. Defuncti; BONA, Rerum liturg., I, xvii, §§ 6-7; HITTORP, De div. cathol. eccles. officiis, 1329; GUYET, Heortologia, 462-73 (on the rubrics to be observed in the office of the dead); CATALANUS, Rituale Romanum, I (1757), 408, 416 etc.; CERIANAI, Circa obligationem officii defunctorum; BÄUMER-BIRON,Hist. du Brév., II, 30, 37, 131 etc.; BATIFFOL, Hist. du Brév., 181-92; PLAINE, La piété envers les morts in Rev. du clergé français, IV (1895), 365 sqq.; La fête des morts, ibid., VIII (1896), 432 sqq.; La messe des morts, ibid., XVI (1898), 196; EBNER, Quellen u. Forschungen zur Gesch. des Missale Romanum, 44, 53 etc.; THALHOFER, Handbuch der kathol. Liturgik, II (Freiburg, 1893), 502-08; KEFERLOHER, Das Todtenofficium der röm. Kirche (Munich, 1873); HOEYNEK, Officium defunctorum (Kempten, 1892); IDEM, Zur Gesch. des Officium defunctorum in Katholik., II (1893), 329. See also the literature of the article BURIAL and other articles cited above, CEMETERY, CREMATION etc.