This biblical writer liked to clinch his argument by appealing to Scripture. [source][source][source]
"His argument up till now has been the negative one that the animal sacrifices of the old covenant were unavailing. Now he says positively that Christ"s sacrifice, which established the new covenant, was effectual. It really put away sin. And it was foreshadowed in the same passage from Jeremiah." [1][source]
The passage he quoted first ( Psalm 40:6-8) expresses Messiah"s commitment to offer His body as a sacrifice to God (at His first advent) because animal sacrifices of all types were inadequate. God"s will was the perfection (i.e, thorough cleansing) of believers. Jesus was not some dumb animal that offered its life unthinkingly. He consciously and deliberately offered His life in obedience to God"s will. [source][source][source]
"The psalmist"s words, "Lo, I am come to do thy will, O God," sum up the whole tenor of our Lord"s life and ministry, and express the essence of that true sacrifice which God desires." [2][source]
The "role of the book" is the written instruction (torah) of God. Throughout the Old Testament the prophets presented Messiah as committed to doing God"s will completely. [source][source][source]