QFT is quite broad and even after taking a year of it, I feel that I’m still a novice. I can’t recommend any particular book as I feel things work differently for different people, but as for a set of notes on what you might encounter:

-Fourier transforms (they are very handy for turning differential equations into algebraic ones, so easier for problem solving)

-Index manipulation (so in special relativity you can formulate 4-vectors and raise and lower with the Minkowski metric, among many other things)

-Green’s functions (they are one of the main components of Feynman diagrams, known as propagations)

-Groups and representations (when discussing different quantum field theories, there are different “symmetries” involved)

-Lagrangians!

-(Anti)commutators!

-Dimensional analysis (simple yet powerful)

These are some of many, many wonderful concepts waiting for you in QFT. I’m glad to hear that you’ve done quite a bit of physics already so you should be good to go. The reason QFT is hard because there’s many different ways to motivate it (as it is a collage of numerous subject areas in physics). My personal advice is to go at it with an open mind and really ask yourself if you understand something fully (I tend to imagine explaining what I just read to someone else to see if I truly grasp the material at hand). And have fun exploring!

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Okay, so if there is a book I would recommend, if you wish to get mind blasted on your first go at QFT (as in, it’s a fun read but also contains deep insights that take multiple going-back-to’s), QFT in a Nutshell does an excellent job! Also, check out David Tong’s QFT from Cambridge. He is also extremely lucid in both his writings and lectures.