What is the the "Book of Truth" referenced in Daniel 10:21?

The accurate translation of “Bechtav Emet” is “in a true script”. “Indeed, I shall tell you what is inscribed in a true script” (Daniel 10:21). The Angel told Daniel what is inscribed in a true script in Heaven, which is unchangeable. This is referring to the loss of the kingdoms and the salvation of Israel, as stated in the following chapter. (See Metzudat David on Daniel. Talmud Yevamot 105a with Rashi).