There are 3 books to be found in the Reliquary:
How to be a Master Swordsman: Go down the steps inside of the Brightwall Academy to enter the Reliquary and right away turn right once inside. It will be to the right on a stand.
Famous Killers: Carl Tendency: You will first need to complete the “An Ancient Key” quest to reach a locked library inside of the Reliquary. It will then be on a stand in the corner of the room.
The Book of Doom: This will be the final book in the “Pen is Mightier Than The quest. You will need to find and return all 29 books before Samuel gives you the key to this final part of the Reliquary that has this book.
Once you accept the quest, enter the first locked door on the right and then go down the stairs till you eventually fight some hollow men. Then once you have fought them all off, head down the hallway to the lowermost floor in the collapsed area. Go past the rubble and you will see the book on a pedestal to the right.