Textual analysis: Following modern divisions of Isaiah, we have inserted a chapter division in Jacob's sermon. This division was not part of the original oral sermon, nor was it a division made in the first edition of the Book of Mormon. Our current verse 1 of chapter 8 came immediately after the text of our current 2 Nephi 7:11.
The earlier texts of Isaiah did not have such divisions either:
"It should hardly surprise us that Nephi's and Jacob's quotations of Isaiah in the ancient text of the Book of Mormon do not break at our current chapter and verse designations. The Isaiah Scroll of the Dead Sea scrolls, as well as Greek and other ancient biblical manuscripts, show that chapter and verse breaks were not present in ancient manuscripts." (Gee, John. "Choose the Things that Please Me": On the Selection of the Isaiah Sections in the Book of Mormon." In: Isaiah in the Book of Mormon. FARMS 1998, p.68).
Therefore, in order to adequately understand verse 1, we must remember the text which preceded it. At the end of 2 Nephi chapter 7, Isaiah is addressing the hypocrites of Israel. In verse 1 of chapter 8 he shifts the locus of discussion, and now addresses "ye that follow after righteousness." The current split in chapters occurs in the middle of a literary shift in the group to whom the address is directed.
Scriptural analysis: Isaiah has made a distinction between those who appear to be righteous, but who are not, and those who are following after righteousness. Because the hypocritical group might appear to be following righteousness (they are walking in light - but the light of their own fires: 2 Ne. 7:11 "Behold all ye that kindle fire, that compass yourselves about with sparks, walk in the light of your fire and in the sparks which ye have kindled. This shall ye have of mine hand--ye shall lie down in sorrow.") direction must be given to those who would follow righteousness. To follow, they must know where to look.
Isaiah sends them to their heritage. In poetic language, he sends them to the "the rock from whence ye are hewn, and to the hole of the pit from whence ye are digged." These are two parallel forms that serve not as literal, but figurative examples of where they are to turn. In each case, the reminder is that they are a part of something that has had an existence, and it is to that existing past that they are sent. The particular direction becomes explicit in verse 2.
Literary note: While the parallel imagery of sources is apparent in the King James version of these verses, they nevertheless miss some of the poetry of the parallel. In Gileadi's translation (and others) these phrases are rendered:
"Look to the rock from which you were cut, to the quarry out of which you were hewn." (Gileadi, The Book of Isaiah p. 197). The image of the quarry is a much tighter tie to the cut rock than the "pit from when ye are digged."