The game's class storylines have endings to them, and there's only one because light/dark alignment doesn't affect the ending. So I wouldn't expect that the book would be an ending to the game, but it might be an ending to either the second war or the Sith Empire itself.
I was kind of hoping that the Sith Empire would exist to form continuity with the Sith in the New Sith Wars. I suppose that theoretically speaking, that would be the case no matter what since Darth Ruin is supposed to unite all the miscellaneous Sith cults worshiping Revan, Exar Kun, the Mecrosa Order, and presumably any holdovers from the Sith Empire should it collapse, but it would be cool if it existed as an entity like the Imperial Remnant until that time.
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It's plausible that the book could be a sequel to Revan since his fate is extremely ambiguous (he vanishes in a flash of light), but personally I think if that's the case it's wonky to the extreme and is basically deifying Revan to an absurd degree.