A thorough search around the base of the bookstand where you found Agrius’ Journal can also turn up the key to Halldir's Tomb, though it's really unnecessary- none of the locks in this dungeon are even Expert level, nevermind Master, though you can pick up some Lockpicking experience as you go.
Loot the corpses if you like- the gear is nothing special, but some of it is salable and there are some coins and lockpicks to be had. Then, once you're finished taking what you like from the room, open the door near the bookstand and continue inward.
More mess and mushrooms wait for you, and then an upward spiral staircase. Ascend with moderate caution- while nothing is waiting to ambush you at the top, there are a few things in hearing range if you're particularly loud.
At the top of the staircase is an intersection of tunnels- facing it is a collapsed area with water in, devoid of anything useful. To your left you'll find an embalming table with some ruined books and occasionally something small worth snagging on your way past. The remaining direction, however, leads up a bit more- and two ghosts wait at the end of this hallway. If you're not extremely loud they won't notice you at first, so archers can easily get the drop on them- and they aren't terribly powerful, just a pair of melee fighters.
Slay the two however you will and then make your way through the door behind them- carefully. Another pair of ghosts wait just on the far side of the door, seated in thrones that bracket it on the same wall. This is where the real challenge starts for sneaky characters- if you're cautious and quiet, they won't show themselves until you enter the room entirely. Amusingly, you can pickpocket them even if they're still invisible, and using sneak attacks to kill them is very quick and efficient. Warrior types will likely want to go through the door backwards so as to be able to strike at them immediately. Be wary if you do this and don’t move around into the new areas, or you might trigger a nearby trap that can make the fight a bit more complicated.
