If you'd like to read a refresher on the calculus of multiple variables, I think you'd enjoy Div, Grad, Curl and All That. It's in the same sort of informal st

If you'd like to read a refresher on the calculus of multiple variables, I think you'd enjoy Div, Grad, Curl and All That. It's in the same sort of informal style of Feynman's Lectures. It's not rigorous like Rudin, but it gives you a good intuitive understanding of the subject.