Bring the Lich's Spellbook to Bethor Iceshard in the Undercity.
There is a lich who dwells on the island in Brightwater Lake to the north. Although he is a free-willed Forsaken, he believes all other Undead are Scourge slaves and has his minions attack anyone who approaches him.
But because he is Forsaken, the Queen will want him. And if he is skilled in Necromancy, then his knowledge would be...quite useful to us.
To discover his identity and his talents, I must see his spellbook. Steal into his camp and acquire it, then return to me.