The MT of Daniel 10:21 is:
אֲבָל֙ אַגִּ֣יד לְךָ֔ אֶת־הָרָשׁ֥וּם בִּכְתָ֖ב אֱמֶ֑ת וְאֵ֨ין אֶחָ֜ד מִתְחַזֵּ֤ק עִמִּי֙ עַל־אֵ֔לֶּה כִּ֥י אִם־מִיכָאֵ֖ל שַׂרְכֶֽם
The language is late Biblical Hebrew prose, whose grammar is relatively close to modern Hebrew.
The words in question are in bold, transliterated as b'ctav emet.
emet is "true". In the earlier books of the OT, emet is always in noun form, "truth" or "righteousness". It is only in the books written in later Hebrew; Joshua, Jeremiah, Ezekiel, Malachi, Chronicles and one instance in Proverbs, where emet is clearly used as an adjective. Some examples are:
Malachi 2:6 (NIV):
True instruction was in his mouth and nothing false was found on his lips...
Ezekiel 18:8 (NIV):
He does not lend to them at interest or take a profit from them. He withholds his hand from doing wrong and judges fairly between two parties
c'tav is the noun form of the verb ctv, to write, which appears as a noun only four times in the OT, in the later books Daniel and Chronicles I and II, each time with the prefix b, "in" as in Daniel 10:21. The other instances are:
I Chronicles 28:19 (NIV):
"All this," David said, "I have in writing as a result of the LORD's hand on me, and he enabled me to understand all the details of the plan."
II Chronicles 2:11 (NIV):
Hiram king of Tyre replied by letter to Solomon: "Because the LORD loves his people, he has made you their king."
II Chronicles 35:4 (NIV):
Prepare yourselves by families in your divisions, according to the instructions written by David king of Israel and by his son Solomon
In these instances and especially in Jeremiah, c'tav has the meaning of "writ" or "decree" in modern English.
So it appears to me that the best translation into modern English of these words in Daniel 10:21 is "the true decree", that is, a decree transcribed by a (here unspecified) prophet by divine command, as in the situation described in Jeremiah 30:2 (NIV):
"This is what the LORD, the God of Israel, says: 'Write in a book all the words I have spoken to you.
I see no support in the OT for understanding the c'tav emet in Daniel 10:21 as referring to a specific book called the "Book of Truth" as translated in the NIV. It would be ill-advised to start searching for such a work in the Cairo Genizah or in the Judean desert.
I also see no support for the NIV's use of "a book" in Jeremiah 30:2 cited above. Although the Hebrew word used is ספר sefer which usually means "scroll", in this context it means "on parchment", in order to give official status, as opposed to writing on shard as in the case of the Lachish letters for more casual writing, or on papyrus, which was not as durable.
This interpretation is consistent with the RASHI on Daniel 10:21.