"The one" implies that the others hadn't written an important novel. Your ignorance doesn't enter into it because that's stated in a second independent clause. One of the writers came into the room, the one who had written an important novel that I didn't know about.
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The others either hadn't written important novels or they had and you knew about those.One of the writers came into the room, the one who had written an important novel that I didn't know about.