I have stolen the nobleman/officers book of records but am
I supposed to say something specific to Barg?

you also have to get the other two pieces of evidence. talk to the guy at the docks, and i cant remember the other. somebody tells you where to go tho so it should be in your journal. IRL you could also take the preferable option os killing the entire roenall family to eliminate their vile stench from the worls, but thats not an option and woudnt get Nalia out anyhow.its just occured to me that the other guy to talk to might be in the seas bounty, but im not sure.

Yes the person you wnat to talk is located at the Sea's Bounty Inn at the Docks. IIRC he will in turn lead you to someone else.