Oh, no, she was referring to a candy bar. He said that he always wanted the Clark bar because they reminded him of his childhood, and she said she preferred a Zagnut. He said that no one "prefers" a zagnut. Zagnut is a kind of candy bar. I think it's a chocolate bar coated in peanut butter, but I'm not sure.
And, for those who aren't huge fans of the books...the gentleman that Reacher bumps into at the diner as he is walking in near the very end of the season is the author, Lee Child.