Video from The Bible Project
"The Angel of the Lord is one of the most fascinating spiritual beings in the Hebrew Scriptures. Every time this figure appears, he’s depicted as if he is God, but also an angel sent by God. In this video, we explore this paradoxical character and discover how he prepares us for the grand claims that will be made about Jesus in the New Testament." from video introduction
The exact identity of the “angel of the Lord” is not given in the Bible. There are however many important “clues” to his identity. We find Old and New Testament references to “angels of the Lord,” “an angel of the Lord,” and “the angel of the Lord.” When “the” is used, it is referring to a unique being, separate from the other angels. The angel of the Lord speaks as God, identifies Himself with God, and exercises the responsibilities of God.
An excellent book on the subject is: The Angel of the LORD: A Biblical, Historical, and Theological Study by Douglas Van Dorn