This is a story based achievement for the Dragonborn DLC. Once you've got the bend will shout, you'll be sent to master Neloth at Tel Mithryn and he'll ask for your help getting a black book from Nchardak.
Once inside, you'll need to collect 4 control cubes.
The first is easy to find in the Nchardak Great Chamber.
The second is in the Nchardak Workshop powering a flame defense.
The 3rd is in a room that has been flooded. Use the two cubes you have to drain the water and proceed to the bottom and along the now accessible corridor and retrieve the cube, go back up to the top and retrieve the 2 cubes used to drain the room.
The forth will now be accessible in a room to the south.
Now go back to the great chamber and use two of the cubes to drain the water , this will gain you access to the aqueduct.
The puzzle here involves operating the red control pedestals to lower the bridges and proceed in. Operate the left and right pedestals (no particular order) to set the bridges in the right order. This will lead you up to a blue pedestal to drain the rest of the room. Neloth will volunteer himself to stay behind to claim the last, leaving you to brave the swinging blades and traps to claim the last cube.
Swim back to the Great Chamber, remove one of the cubes from the draining systems, and operate the boilers. Return to the Reading Room after that and you, or Neloth, can operate the switch to unlock the book