Horatiu Nastase's Introduction to Quantum Field Theory is primarily available as a graduate-level textbook published by Cambridge University Press

Moving beyond canonical quantization, the text explores Feynman’s path integral approach. This is crucial for understanding gauge theories and the functional methods used in modern particle physics. Seeking the PDF: What to Look For

Nastase’s book is dense. Pair it with free video series like Tobias Osborne’s QFT lectures or David Tong’s QFT notes (available on DAMTP). Use Nastase as the "deep reference."