1st Floor, past the 100 Lock/Security door on top of the safe. (Or sneak up to the 3rd floor and get the key from the suitcase next to the skeleton). The room can also be accessed through a hole in the floor of a room on the west side of the 2nd floor (Must pick the Hard (75) lock on the door).
Right before you enter the vault door turn around and jump down to the pipes below the walkway, there is a duffle bag with the book inside. The only way back is to go east from the bag and jump on the valved pipe then back to main platform.