It can be found triangulated Northeast of Camp Sentinel and South of the Depleted Ore MIne.

Once inside you'll find a dead priest and by him a Book of Notes and a key. Upon reading it you find that the bandits are after something, but he does not know what and obviously never finds out.

Down some stairs you will find a group of bandits one name Sonja who drops another key.  After the fight you can go right or left, if you go right you find a barrier in the way and three bandits on the other side.

To the left you find a set of stairs and 4 pressure plates with 2 chests on the other side of them. The chest on the right is pickable. The chest on the left is opened by the key you picked up near the priest. The pressure plates are what brings down that barrier spell. you need to step on them in the right order until all are glowing blue. If you do the wrong one the puzzle resets.

Once the barrier is down you go back to that main T intersection and take that right turn going down the stairs. A conversation will start with the bandit leader Ragon. A fight ensues where he drops a ring for the A Hunting We Shall Go Again quest and random legendary gear.

Afterwards there will be 3 golden dragon statues. The 2 that are at the entrance to the hallway must be rotated until they're facing away from each other. The 3 which is up and to the left when facing those 2 statues needs to be rotated until its facing the hallway. The "Masters Spell Book" for the Borrowed Book quest appears. You can pick it up, loot the place, and then you can leave.