What a great book! I hope you enjoyed it, too. As far as "boomeranging" in this novel, Ellison describes the world moving as a boomerang, instead of moving forward like an arrow. Our individual lives move forward each day, but the future and progress of our world is greatly affected by its history that does not simply stay in the past. The path of a boomerang always ends up back to where it originally started, just like the book. Ellison begins and ends the story in the same place, underground.
The narrator in the novel pushed forward in life to educate himself and help to rid the world of racism and social prejudices. As much as he grows as a person and learns to love and accept himself, he finds that society will not. He ends up where he started with society not seeing him for who he really is. As a result, the narrator has moved forward, but society has "boomeranged" around him.