You should have gotten a quest from one of the librarians (find out why the lava smells so bad). After solving that quest you will find the Head Librarian way to the East and North inside the library.
As for Sigurd, you should have gotten the quest from him first before killing the other Assassin. If you did have his quest first, then you should find him inside his house (you did mark the correct house on your map right?).
I chose not to kill the other assassin after he told me Sigurd was an assassin too and also a liar. This left me with two open quests in my log and the second fellow disappeard from his cellar. This time around (second time in Act 3) I'm putting off making the decision.
What's the best way to do this? I usually don't like killing quests but in this case I ended up wanting to kill them both.
