1st way:
At Windhowl talk to Lord Sunrock and say Jarko. He will mention that he wants his spellbook. Go to Jarko Caves at 3rd level and search for Jarko the Necromancer (he looks like a human mage).
Kill him and take the spellbook that he will drop. Take it back to Lord Sunrock and say Spellbook. He will reward you with a Golden Ring and 300 gold pieces.
2nd way: When you kill Jarko and pick up his spellbook go to Lighthaven and locate the NPC Araknor (he is behind -east- the temple).
Say Jarko, Spellbook and he will reward you with a ring of darkness or a ring of confidence and the ability to learn some necromantic spells later from Blindman (Xanth) (become his apprentice, consult the notes on the end of the Spells Chart for more information).
Note 1: An easy way to kill Jarko if you are a fighter (or if you use melee combat) is to wait until Jarko go to a corner of his room and then attack him.
Many of his summon will appear at the other side of the wall and will not disturb you in your work :) On the other hand, if you are a mage, use an earth/fire based spell for the damage and Lesser Drain for recovering your lost HP.
Gain:
Note : You can do this quest as many times as you want.