I'm learning QFT by reading Lewis Ryder's book, so my question in short is: how he arrives at Eq 7.57 and Eq 7.58? If you don't have the book, the question is: why are there Minkowski metric terms in the Feynman rules for a gauge field coupling to itself? If the answer is complicated, simply tell me where I can find it please, I have Peskin's and Weinberg's book, too. For me it seems impossible to proceed after Chapter 7 with this book, it doesn't have a systematic way of introducing Feynman diagrams, and the Feynman rules for vector bosons seem to be pulled out of a hat, is this a suitable book for a man learning QFT for the first time on his own? Thank you.