This Dark Endeavour by Kenneth Oppel

While exploring their mansion one day, twins Victor and Konrad stumble upon a place known as the dark library where they find some interesting reading, and subsequently get warned by their father to stay away from that room. When Konrad falls ill, Victor, their beautiful cousin Elizabeth and their friend Henry become involved in a search for three ingredients needed to make the Elixir of Life to save Konrad. Suspense, interesting characters, romance, and peril all combine to make this a thrilling read.

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