Golden Book Symbol
The first list that the angel brings is in an ornate gold book, which shows that a great deal of attention has been paid to its outward appearance. This is a symbol of the people who's names appear on the list within it; the list details all the people who love God. The fact that the book is so concerned with outside appearances symbolizes the way in which the people on the list are also more concerned with being seen to love God than actually manifesting God's love in the way their live their own lives.
Second List Symbol
The second list that the angel brings shows the names of those who are blessed by God. This is not in an ornate, outwardly-swanky book, which shows that the contents of the book are more important than it's outward appearance. This symbolizes the fact that God blesses those who strive to demonstrate their love for him from the inside out, and actually love those whom he has created.
Peaceful Sleep Symbol
Abou Ben Adhem awakens from a deep and peaceful sleep when the angel appears to him. This symbolizes that he is a good man; nothing is keeping him awake and he has no guilt about anything preventing him from sleeping. This is a symbolic character endorsement from the speaker of the poem.
Angel Visitation Symbol
Angels are emissaries of God, so the fact that one has chosen to visit Abou Ben Adhem not once, but twice, on consecutive nights, symbolizes his importance to God, and his importance to mankind in showing how one should live, and love one's fellow humans.
"May His Tribe Increase" Symbol
The speaker, in an aside, says "may his tribe increase" which is a symbol of Abou Ben Adhem's goodness, in that the world needs more men like him, a character endoresement and a symbol of the fact he is what is "right" about the world.
