How can I access the Nightmare difficulty in the Quake TC "X-Men: The Ravages of Apocalypse"?

To open the Nightmare difficulty, one must find a small hidden black book in the room named 'The Library'. This is on the ground floor of the X-Mansion, through the rightmost door behind the staircase. The black book is a very small switch found embedded in the bookcase immediately to the player's right as they enter the library. Walk into it and the Nightmare selector in the Women's Dormitory will be opened. If you can't find the book - try running across all the walls of the Library until you find it.