At the screen were Gabriel shows his face (after opening the book), before you press space to continue you can input a cheat to unlock a "Cheats" option in the Extras section of the pause menu.
For the Xbox controller you must use the D-Pad buttons as directional arrows: Up(2), Down(2), Left, Right, Left, Right, B, A.
For keyboard controller use the directional arrows: Up(2), Down(2), Left, Right, Left, Right, ESC, Space
If you input the code correctly instead of Gabriel shows you his vampire glowing eyes he will wink at you.
* enabling cheat options will prevent achievements from being earned and the game from being saved.
(supplied by: Amanpia)