You have to accept a quest to get into the basement of the Vermin Control office. Just keep talking to the guy and see if he'll ask you to do something for him.
I'm certain that the Tome of Bone and Ash will let you disarm the Armored Skeletons at the Mortuary. (You get 800 experience points for each one, which is very nice.) I don't know of any way to disarm the guards who guard the Silent King's hall.
To get the fingerbone from Marrow-Fiend, you have to do some things that a normal person would never do. But the Nameless One is not exactly a normal person, is he? Lots of people refer to him as a "walking corpse", including Marrow-Fiend. Do you see where this is going? Marrow-Fiend likes to eat corpses, and he gets hungry just looking at the Nameless One, so...
There is an NPC Priest in the game who can join your party, but the Nameless One cannot become a Priest. Take your pick between Fighter, Mage, or Thief, or try a couple or all three. One of the nice things about switching classes in PS:T is that when you switch to a Thief or a Mage, you retain all of the benefits of being a Fighter (although your weapons are limited to those useable by a Mage or a Thief)--THAC0, weapon specialization, etc. I put as many points as I can into Fists and Daggers. There's something really cool about a character who fights hand-to-hand and wears rings and tattoos instead of armor.
One thing you need to be aware of is that you get a specialization bonus when you reach 7th Level. The benefits are different for each of the classes. You can also get a specialization bonus at 12th Level, but ONLY if you advance to 12th Level in the same class that received the 7th Level specialization bonus. You'll want to get both bonuses. If you got the Fighter's 7th Level bonus, go ahead and become an 11th Level Mage if you desire, but switch back to a Fighter and advance to 12th Level before you level up to 12th Level as a Mage. Check out your stat increases, and look at the new tattoos that will appear in Fell's shop.
Here's a big hint. Annah can train you to be a Thief. In other words, you can have a trainer in your own party. If you want to become a Thief, just talk to her. There's also an NPC in the game who is a Fighter/Mage. Guess what that means?

The Priest NPC cannot train you to be a Priest, but she has other redeeming qualities.
The question you should ask is, what do you want the Nameless One to be? There are places where you can earn unlimited experience, and you can change classes any time you want. But keep this in mind: the Nameless One is on a voyage of self-discovery, and his class is not the most important factor in his development. I think your focus should be on learning as much as you can about EVERYTHING in the game. Enjoy it.
