For Whom the Bell Tolls
Karkov's the most intelligent guy Robert Jordan's ever met, and if that's not a qualification for mentor, we don't know what is. It's really Karkov who's responsible for the transformation of Robert Jordan from a zealot with a puritanical dedication to the cause into the more disaffected, realistic, even-minded guy we know. Karkov tells it like it is, and he wants to make sure Robert Jordan learns from him. Karkov's mentorship occurred before the events of the book, but does that really make a difference? It's Karkov that Robert Jordan wants to talk to right before he dies about everything he's learned, and it's pretty clear that he perceives the "education" the few days with Maria and the guerillas have given him as a continuation of the education Karkov gave him.