I just finished reading QED and now I want to learn the science behind what Feynman talks about. In the introduction by A. Zee, he mentions his book "Quantum Field Theory in a Nutshell", saying that it is the next step for someone interested in learning more. I went to a bookstore, looked at the first chapter, and realized that it was above my level. What books would be good for me? For reference, I've taken an intro to mechanics and electromagnetism along with the entire calculus course progression, (I can deal with the math in most books, it's the science I need to learn). I hope this is the right place to be asking this!