N.B. This piece is about the experimental lectionary I am putting together (to peruse the lectionary, see the Sarum Office page and follow the link at the bottom of that page).
Isaiah provides the traditional lessons for Advent, which can vary from 22 to 28 days. (Last year there were 28 days because Christmas fell on a Sunday, giving a full fourth week after the Fourth Sunday).
Some of Isaiah is not particularly relevant to either the first or the second coming of Christ: there is a whole historical section about the attempted Assyrian subjugation of Jerusalem in the reign of King Hezekiah, for example; and there are also the parts, like Isaiah chapter 53, that are part of the liturgy of Holy Week because of their prophetic witness to the crucifixion. Parts of Isaiah already form the Matins lessons of Christmas and Epiphany.
Once one has taken out these parts, the remaining "Advent Isaiah" fits fairly neatly as a continuous lectionary into Advent, if one reads two sections each day: one section at Matins, and one at Vespers. There are occasional ferial days, such as the Ember days, when the Matins lessons are occupied by a Gospel Homily - on these days one can shift the first Isaiah lesson to become the lesson after Prime.
There is one slight complication to the neat continuous reading of Isaiah in Advent: which is O sapientia and the O antiphons, beginning on the 16th December in the Sarum rite. I therefore made a special series of lessons for 16th to 23rd December, and I tried as best I could when constructing the lectionary to make the lessons coincide with the O antiphons. By interrupting the continuous lessons and jumping around a little, it wasn't difficult - for O oriens, for example, there is the very obvious "Arise, shine" of Isaiah chapter 60. I don't think that the lack of continuity in reading through Isaiah is very important, however - aside from a few sections in Isaiah there is no particularly clear reason for the consecutive ordering of much of its material.
A couple of things stood out for me when putting the lessons in place over the last few years.
Firstly, once one sets a sequence running (e.g. for the O antiphons), the biblical text almost seems to be ready made for the series. The final O antiphon O virgo virginum to take one sample of many, just happens to coincide naturally with the final chapter of Isaiah, chapter 66. Read it: the wealth of symbolically relevant Marian detail is astonishing. Verse 7 stands out as an anticipation of the traditional understanding of the mode of our Lord's birth from the Virgin in such an obvious way that I cannot believe that the text had nothing to do with the development of the traditional idea.
The second thing that stood out for me was the general lack of reference in Isaiah to David, to his throne, to the rule of the monarchy of Israel in general. Take out the well known prophecy of chapter 9, "Unto us a son is born", and almost the only other mention of David in the book that does any symbolic or prophetic work, so to speak, is the phrase "the sure mercies of David" in chapter 55. Odd, in a way: there is a lot more Davidic material in Jeremiah and other prophets such as Zechariah - yet here are all those purple passages about the nations flowing to Jerusalem and the Temple, the restoration of all things to a primitive Eden... and yet the role of the house and throne of David in all of this is barely mentioned. Even the figure of Cyrus seems much more central for a goodish part of the book. I am not sure what to make of this, nor where to turn for really good critical studies of Isaiah. Perhaps someone could be kind and enlighten me.
Isaiah forms the bulk of the Advent lectionary, and the remaining lessons for Prime and the second lesson for Vespers are provided by two fairly obvious Advent texts, Daniel and Revelation respectively. The Apocalypse draws heavily on Daniel, so the two are complementary. I read in The Name of the Rose that the Benedictines read Revelation at Prime, although I have never been able to corroborate this - but it does seem like a natural choice for Advent, ending with the descent of the Bride and the invocation of the Bridegroom, just as he is about to "leap down from heaven" in the silence of the night. Daniel is not only complementary for the theme of Advent, but also runs on from the November lessons - my adjustments to the Sarum lectionary means that Daniel runs straight on from Ezekiel in November, the other minor prophets fitting into November also. This provides a continuity that extends the theme of Advent that one finds in the minor prophets (such as Zechariah and Malachi) back into November, which is only fitting given that Advent once began after Martinmas. I believe that it still does in Milan.