This Dark Endeavor - Kenneth Oppel

Bravery, danger, and intense passion. How does obsession begin?
Victor and Konrad are the twin brothers Frankenstein. They are nearly inseparable. Growing up, their lives are filled with imaginary adventures...until the day their adventures turn all too real.
They stumble upon the Dark Library and discover secret books of alchemy and ancient remedies. Father forbids them from ever entering the room again, but when Konrad falls gravely ill, Victor is drawn back to the Dark Library where he uncovers an ancient formula for the Elixir of Life. Victor, along with his beautiful cousin Elizabeth and friend Henry, immediately set out to find a man who was once known for his alchemical works to help them create the formula.
Determined to save Konrad, the three friends scale the highest trees in Strumwald, dive into the deepest lakes, and even make an unthinkable sacrifice in their quest for the elixir's ingredients. And as if their task was not complicated enough, a new realm of danger--that of illicit love--threatens to end the ordeal in tragedy.