Tome on the Dark Knight in Death's Gambit is a Collectible. Collectibles in Death's Gambit are items that have its effects activate as soon as the item as found.
In the darkness his eyes myriad, glinting deathly orbs with a savage appetite for blood. Take heed of the wary sensation cloying at your nape."
Tome on the Dark Knight Location
In the entrance of Darkness Falls, after going through the first hallway and reaching the ladder going down, under the entrance hallway is another hallway but is not visible. Simply jump from the ladder, land on the hallway, avoid the poison gas and at the end is the tome.
Tome on the Dark Knight Notes & Tips