What does Daniel 10:21 mean?
Daniel's earlier vision was troubling (Daniel 10:1) and apparently caused him to mourn (Daniel 10:2–3). Comfort came in the form of an angelic messenger (Daniel 10:10–14), probably Gabriel. The response was delayed for three weeks because of an entity called "the prince of the kingdom of Persia" (Daniel 10:13–14, 20). The angel is to explain the last vision in greater detail (Daniel 11:2). Only when Michael—the "archangel" (Jude 1:9; Revelation 12:7)—came to help was this angel able to come and minister to Daniel. Michael seems to have special responsibilities regarding Israel (Daniel 12:1).The angelic speaker promises that what he says comes from "the book of truth." This is not necessarily a literal collection of pages. However, it does evoke the idea of God's ability to remember every detail and decision. For example, Jeremiah 30:11 indicates that the Lord recalls and remembers all His acts of discipline towards Israel. Revelation 5:1–5 mentions a seven-sealed scroll which only Jesus can open; this contains a series of judgments. Other symbolic "books" contains records of individuals' struggles (Psalm 56:8) and beginnings (Psalm 139:16).
God's words are always truth (2 Samuel 7:28; Numbers 23:19). Because of this supernatural encounter, Daniel would learn at least some of what the Lord God has in store for the nation of Israel. That description begins in chapter 11, after a comment (Daniel 11:1) that seems to fit better with the end of this chapter.
