I always felt that the Feynman Lectures were really, "an incredibly deep understanding of freshman level physics, targeted towards physics graduate students." (As opposed to freshman level physics targeted towards pre-med students.)

Even after I got my undergrad degree in physics I would reread the Lectures and would see things on a whole new level than I ever understood them the first time.

Edit: even "Surely you're Joking" had jokes that only someone that had graduate level general relativity would get (talking about "gee-mue-nue, gee-mue-nue")