The Apprenticeship of Victor Frankenstein Series 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.

When his grieving father orders the destruction of the Dark Library, Victor retrieves a book in which he finds the promise of not just communicating with the dead, but entering their realm, and soon he, Elizabeth, and Henry are in the spirit world of Château Frankenstein, creating and growing a body.[Book two of The Apprenticeship of Victor Frankenstein series.]