The Apprenticeship of Victor Frankenstein duology by Kenneth Oppel

Victor Frankenstein leads a charmed life. He and his twin brother, Konrad, and their beautiful cousin Elizabeth take lessons at home and spend their spare time fencing and horseback riding. Along with their friend Henry, they have explored all the hidden passageways and secret rooms of the palatial Frankenstein ch�teau. Except one. The Dark Library contains ancient tomes written in strange languages and filled with forbidden knowledge. Their father makes them promise never to visit the library again, but when Konrad becomes deathly ill, Victor knows he must find the book that contains the recipe for the legendary Elixir of Life. Additional content in This Dark Endeavour includes pages from Victor Frankenstein's sketchbook.

Such Wicked Intent continues the story of Victor Frankenstein. Tragedy has forced him to swear off alchemy forever. He burns the Dark Library and vows he will never dabble in the dark sciences again—just as he vows he will no longer covet Elizabeth, his brother’s betrothed. If only these things were not so tempting. When he and Elizabeth discover a portal into the spirit world, they cannot resist. Together with Victor’s twin, Konrad, and their friend Henry, they venture into a place of infinite possibilities where power and passion reign. But as they search for the knowledge to raise the dead, they unknowingly unlock a darkness from which they may never return.
 
- Goodreads Summary