True, the game is after the books, but books and mysteries uncovered in the game are likely already known to someone who's read the books. At least, that how it appears to be.

That's right. You'll know from the beginning the true nature of the Wild Hunt, who they are after, or why everybody is surprised Geralt is alive. Also, that Triss isn't truly as nice as she appears or Geralt doesn't try to sleep with every female he comes across. But they mostly nailed the atmosphere of the books, and It just feels great seeing that world coming alive.