A walk-through Edit
I just completed this quest aliance and got my "poison" skill. You go to Stormwind city and talk with Methias Shaw in SI-7. He gives you the "Mathias and the Defias" quest. This is the only quest you need to get the poison skill.
1. travel to westfall past Sentinal Hill traveling east toward duskwood. Just before the bridge turn right and go along the bank until you see Agent Kearmen on your mini map or see Kearman jump down from a tree in stealth mode. Speaking to Agent Kearman completes this part of the quest.
2.The book you receive gives you a hint to the MOPs weakness (backstab) but doesn't apply to the spiked hair guy so don't try. You will die, he hits over 100 a pop. Just stealth up behind him and pick pocket the "key." The key is a quest item and is not used again. If you get the book first you will not be able to stealth anywhere near this guy without him going aggro and killing you.
3. Now stealth into the tower and you should be able to avoid all the mops as you climb to the top floor. On the top floor is one MOP elite but his back is to you, so stealth up behind and SAP him. Then turn to the chest that is to his left and use lock picking skills to open it. Grab the book inside.
4. You will be poisoned when you open the chest but ignore and hit speed, jump down to the main floor, and run out toward the river. Head back to Stormwind (there is a flight path to Sentinel Hill) and complete the quest with Methias Shaw. He will tell you to see a doctor to remove the poison but it isn't necessary to get the poison skill.
5. Use a potion or get someone to "remove" poison and you will be healed. Then go to the rouge trainer and buy your poison skill. Enjoy.