For book recommendations, the ones that come to the top of my mind are:
- Griffiths' Quantum Mechanics [0]. It's become a pretty standard undergrad QM text, and in my experience was very approachable.
- Div, Grad, Curl, and all that by Schehey [1]. I don't remember how much into vector calculus the QFT book got, but this one turned the tide of my undergrad personally. For ~120 pages it gave me a better intuition with 3D calculus than any other resource.
- Something that covers calculus of variations, Euler-Legrange equation, etc. I first covered this in Classical Mechanics but don't remember the textbook. The Feynman Lectures of Physics [2] probably covers it, but I don't know for certain. Incidentally, Feynman is all over QFT, so his undergrad materials are probably excellent prep materials.
I don't remember whether the book introduces bra-kets (Dirac notation) or assumes them, and I don't remember if Griffiths uses it at all. I first saw this notation in General Relativity, with Spacetime and Geometry [3], but I think there are definitely better materials that can explain the notation better.
I'm pretty sure all of these books, and plenty more on these topics, should be widely (or freely) available. Good luck :)
[0] https://www.goodreads.com/book/show/153908.Introduction_to_Q...
[1] https://www.goodreads.com/book/show/703104.Div_Grad_Curl_and...
[2] https://www.goodreads.com/book/show/5546.The_Feynman_Lecture...
[3] https://www.goodreads.com/book/show/259680.Spacetime_and_Geo...
Edit: newlines always get me
Indeed it does:
https://www.feynmanlectures.caltech.edu/II_19.html
The original lecture recording is also available:
I know a condensed matter postdoc who told me he felt ready to tackle the Feynman lectures only after he had completed his phd....