"Calm yourself, we're not here for whatever it is you stole. A halfling told us you might know a few things we'd be interested in hearing. There's a drink in it for you if you tell us what you know. More if you tell us something useful. You interested?
Assuming he doesn't do anything unexpected, we'll take him back to the Harvester's Haven, find a seat in the back where his smell isn't likely to bother him, get his drink, and ask some questions. To start...
I heard you've been to the Lost Library. Is this true?
(assuming he doesn't deny it) Do you have any way of proving it?
Where is the Lost Library?
If we showed you one, could you point it out on a map?
What did you see while you were there? Describe it in as much detail as you can.
What did you steal, anyways?
...Anything more that I can think of depends on how he answers those.
I'll leave the charming to Viggo, since he's far better at it and I see no need to intimidate the man, but I'm hoping the promise of alcohol and possibility of "more" should help to loosen his lips a bit.