you need to understand perturbation theory in the context of qft. that's no small feat but since it's a pretty universally studied grad core subject there's a lot of material. this book was actually suggested to me here

https://www.amazon.com/Renormalization-Methods-William-David...

two good (Gentle) senior undergrad books are

https://www.amazon.com/Student-Friendly-Quantum-Field-Theory...

https://www.amazon.com/Field-Quantization-Walter-Greiner/dp/...

and then there's always griffiths

https://www.amazon.com/Introduction-Elementary-Particles-Dav...

note that you don't need to read this cover to cover if you're not interested in actually doing calculations. you can skim.