The letter of the Law. The NT did not exist as a Bible, when that was written. There were thousands of manuscripts, all those letters sent from hundreds of churches all over the world. The "letter" was the Law and the Prophets. It was the OT that offered death, death because of sin, and no one able to keep the whole law. Yes, even the OT is the Word. It was not the words still that "killed". It was the fact of attempting to live without Jesus Christ giving a human the ability to live the law. Most humans either try to live the law without Jesus Christ. Or they live the letter of the NT, without the power of the Holy Spirit. One cannot do as they please and leave out half the process, one way or the other.
A lot I can say but I would go off topic,however think I have to.
Hebrews 10:7
"Then said I, Lo, I come (in the volume of the book it is written of Me) to do Thy will, O God."
Everything that Jesus did and said while on earth, was written in the Old Testament, and that why He said over and over, "Haven't you read...", "It is written...", and so on. Every detail was written about Jesus' coming, how His birth would be and where. All parts of His life, and even to the words that the High Priest that caused His death at the time of the crucifixion. How can you not believe in Christ when God so perfectly outlined the coming Messiah that He would send.
Hebrews 10:8 "Above when He said, "Sacrifice and offering and burnt offerings and offering for sin Thou wouldest not, neither hadst pleasure therein;" which are offered by the law;"
Paul was quoting, as we read from Psalm 40:6-8. What our Lord is saying in that fortieth Psalm is; "I don't want your animals and your sacrifices, but I want your love."
Hebrews 10:9 "Then said He, "Lo, I come to do Thy will, O God," He taketh away the first, that He may establish the second."
God did take away the first [the blood sacrifices and ordinances] when His Son went to the cross. With the death of Christ on the cross the establishment of the second was completed. He took those blood sacrifices and ordinances away because the blood of Christ paid the price for one and all times. God did not take away the law, but the blood sacrifices and those things that were required under those ordinances. No blood sacrifice can sent the Holy Spirit, for it is dead and gone. But at Christ death, the Holy Spirit did come and it warms the heart of everyone that He touches.
Hebrews 10:10 "By the which will we are sanctified through the offering of the body of Jesus Christ once for all."
The blood of animals did not sanctify or make perfect anyone. But the blood of Christ, through our repentance removes all sins from us, and makes us perfect in the eyes of our Heavenly Father. We are sanctified through the Spirit of the One that was sacrificed on the cross.
