I've been trying to remember when we last saw Tharka being all "Devil Bunny" and making some poor sap an offer they couldn't refuse, and I think it might've been back near the middle of "Artifacts," our 26th storyline, when he was first dealing with Dodger...
As for Nick, we first met him back in the middle of "Second Date," our 37th storyline and the first chapter of The City When it Sleeps, the seventh and current book of Daily Grind adventures.
Forty-One - Grand Opening