Korgan's Companion Quest - Baldur's Gate 2
Korgan's Companion Quest starts the moment you recruit him into your party. Before joining your party Korgan tells you that he is searching for a book called the Book of Kaza which is found in the Graveyard District of Athkatla. After recruiting Korgan to your party you'll begin the Help Korgan Recover the Book of Kaza side quest which is the same quest this page will detail.
It's worth noting that the first part of this quest is timed, if you take too long to recover the Book of Kaza then Korgan will leave your party and return to the Copper Coronet. What you need to do for this quest is enter the Lower Tombs map then make your way to the southern portion of this map where you'll find the entrance to the Southern Tombs.
Once you're in the Southern Tombs you'll want to go as far south in this area as possible until you find a small room with a sarcophagus inside of it (pictured below). When you arrive there will be some dialogue with Korgan as he realizes that the area has been looted. Defeat the undead then talk to Korgan again to update this quest.
After updating your quest in the Southern Tombs you'll want to head over to the Temple District for the next part of this quest. In the Temple District our destination is going to be the Pimlico Estate which should be easy to find since it's marked on your map with a map marker.
Inside the Pimlico Estate you'll want to explore both rooms, the bedroom has a couple of bodies in it and the other is empty. Eventually Korgan will mention that the people we're looking for have already cleaned this place out and are long gone. However, Korgan knows where they like to hang out - the Copper Coronet in the Slums District!
Now you're going to want to head over to the Copper Coronet in the Slums District and specifically up to the roof. On the roof you'll find the group of mercenaries that Korgan is after (pictured below) after some clever dialogue you'll be thrown into a fight with them which is quite easy considering they're all poorly equipped and low level.
Defeating Shagbag and his mercenary crew will complete this quest. As for the Book of Kaza, you can find it on the ground in the same location you defeated Shagbag. However, the only thing that you can actually do with this book is sell it to a vendor for 1350G. There are no side quests that involve the book or anything like that - it's merely vendor trash at this point.