This Dark Endeavour by Kenneth Oppel

Victor and Konrad are the twin brothers Frankenstein and inseparable until Konrad falls gravely ill. In the forbidden Dark Library, Victor finds an ancient formula, and seeks an alchemist to recreate the Elixir of Life. With friends Elizabeth and Henry, he scales highest trees in the Strumwald, dives deepest lake caves, and each sacrifices a body part.
 
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