Nazgul-Were they still Men?

[symestreem]

They were once men, great kings of men. Then Sauron the Deceiver gave to them Nine Rings of Power. Blinded by their greed, they took them without question, one by one falling into darkness. They are the Nazgul, the Ringwraiths.

-Aragorn, in the Fellowship of the Ring Movie
Better stop before I quote the whole movie ... But it was a movie quote. Is there anything similar in the books?