I've made a video guide here:
Tome 1 - Right after the pig pens, you'll pass under a bridge and have the option to walk all the way down a long bridge. At the end are some bushes. Walk into them to find a secret area. Make a u-turn to the right as soon as you enter to find the first Tome.
Tome 2 - From that room, leave by going right, under an archway, and across the street through a metal fence. In the back left is a door to a chapel. On the altar inside the chapel is a chest. Inside is the second Tome.
Tome 3 - Once you go inside again after the huge open area, you'll have to climb over a broken door and then go up some stairs. In the next room, there will be another short set of stairs and a door to the right. The tome is in there behind a secret wall. To open the wall, take a left at the top of those stairs and open the chest by the next set of stairs. Then go up those stairs into the next area and under the stairs there to find another chest. Opening those two chests opens the secret wall.
It's hard to explain by text, so I recommend you watch the video!