Note: This side-mission takes place in the Soul Cairn region of the DLC
Look for a soul called Jiub (who can be located South-East from Valerica's location and after telling him that he's dead, he'll ask you to find 10 pages from his book. All of these pages are scattered throughout the Soul Cairn and in return for finding it you'll be given an amulet that boosts your stamina and carrying amounts by an impressive 50 points!
Go to Jiub's campfire. Not far to the south there will be a square, stone-floored area with a purple-glowing well, and a wisp soul circling it. The page is on the floor by the well. There are also a couple of bonemen roaming the area.
On the south-facing side of the fortified wall, run along it heading east. There is a set of stairs adjoining the wall, at the top of which is a soul standing next to a window. Next to the window is the first Opus page.
To the north of Valerica's location will be a small building with a broken Word Wall and a chest. The page is in front of the chest. This small building is more-or-less adjoined to the prison where Valerica is held. This is where Durnehviir later sits.
Starting from where Valerica was found, turn left to go east around the building to the back. Go up the stairs (many souls are up here), and face towards the wall of the fort itself while climing upwards - you should come to a long, covered corridor with a pile of bones at the back next to a chest. The page is in front of that chest. A Wispman appears out of the pile of bones as you approach.
On the south-facing side of the fortified wall, head west to find a small building connected to where the wall forms a corner.Heading into this building and through to the other side leads to a low, sandy-looking area with two purple wells and another small building (with a crystal on top) behind it. At the top of that building, beneath the crystal, is a chest - the page should be in front of it.
Page 6 is nearby the stairs the Dragonborn descended on entering the Soul Cairn. Upon coming down them, look for the second building on the right (see Page 8) - the page is located in there next to a chest. This building is easiest to find by sticking to the path: it forks away to the right near the two souls sitting on the ruined walls.
Start where Page 4 was discovered. There is a path that heads directly eastwards away from that fort. Travel down it, and there should be a purple pool with light coming from it in the distance straight ahead.
Go towards it and you'll see many souls sitting around it like it's a campfire - go beyond them to find a small shrine on a raised stone platform. Head up the steps, and the page will be on floor in front of a lantern.
Like Page 6, this one is near to the staircase you descended on entering the Soul Cairn, only this time it's in the first building on the right instead of the second. This one involves shooting an arrow at both of the glowing orbs next to the doorway - doing this should lower the bars to let you in. Sticking to the same path but heading back to the stairway will take you towards Page 6.
From Valerica's location, turn right and travel west until you reach a building with a crystal on top; there is a soul looking down at a skeleton, and another trapped in a cage beneath the crystal itself.
When you get there, use this location as a viewpoint: look to the north, off to the left, and you should see another crystal atop a more extensive, fort-like building. This is where Page 9 is.
Head there and enter via the steps, travel through the maze-like layout, and find the portal at the back left-hand end of the maze. Jump on the glowing portal which will teleport you onto the roof. The page will be on other side of the chest, beneath the health-draining crystal.
Stand in the gap of the fortified wall facing north-east, look to your right, you will see a blue light, head that direction, there will be a ghost named Morven Stroud next to his broken carriage. The page is on top of a barrel to the left of the carriage.