I used to read Griffiths' electrodynamics book on the school bus.
Since the bus was sometimes infested with spiders, which terrify
me, I also used it to repel a fair few spiders. I'm still really
fond of that beaten up physical copy of the book. I'd only just
heard of vector calculus so working through the examples it seemed
like magic to me. I've loved electromagnetism since then; I ended
up studying plasmas just because they are for me the most essentially
electromagnetic of substances.

I'm still fascinated by what makes vector calculus possible; these
days I mainly use differential forms but I think they tap into the
same part of the world. I still love using delta functions and their
friends too - it seems to me differential forms and generalized
functions are some of the most deeply useful and rooted bits of
maths. One day I mean to learn about de Rham's currents which combine
them - what an excellent electromagnetic name he chose for them! I
had printed out one of his papers on them but it was in French and
I never found the time to (very slowly) read it.

There's other electromagnetism books I prefer now but even thinking
of Griffiths takes me right back to when everything seemed marvelously
odd. I still feel that way about general relativity though; hopefully
there will always be something!