Isaiah 34:16 Look in the scroll of the LORD and read: None of these will be missing, not one will lack her mate. For it is his mouth that has given the order, and his Spirit will gather them together.
4 chapters earlier, in Isaiah 30:8
Go now, write it on a tablet in their presence and inscribe it on a scroll; it will be for the days to come, a witness forever and ever.
Ellicott's Commentary for English Readers
(16) Seek ye out of the book of the Lord . . .--The phrase is an exceptional one. Isaiah applies that title either to this particular section, or to the volume of his collected writings.
Pulpit agrees with Ellicott.
Verse 16. - Seek ye out of the book of the Lord. By "the book of the Lord" some understand a collected volume of Moses and the prophets, psalmists, etc., previous to Isaiah's time, which they suppose to have existed in his day. But there is no evidence of any such collection. It is better to understand the expression of Isaiah's own prophecies, or of such a collection of them as he had made previously to the composition of the present chapter. Nothing contained in the entire book should, he says, fail of its accomplishment. Even the minutiae of the present chapter should, each and all, have their fulfillment, though not, perhaps, in every case a literal one. My mouth... his Spirit. The "mouth" of the prophet and the "Spirit" of God, which dictates to him what he is to write, are in accord; and the Spirit will bring to pass what the mouth inspired by him has "commanded."
Isaiah made the point that he wrote under the inspiration of the Spirit and every word would be fulfilled with nothing missing. He stressed that again in Isaiah 55:11
so is my word that goes out from my mouth: It will not return to me empty but will accomplish what I desire and achieve the purpose for which I sent it.