Hello! I just started reading a book about QFT by Peskin (it was recommended by one of my physics professor and I saw that MIT course on QFT also uses it). However they start right away with Klein-Gordon equation suggesting that I should be familiar with it. I took 2 classes on quantum mechanics (in which I finished the Griffith book) and I was mentioned about Klein-Gordon equation a bit (just that it is a relativistic form of Schrodinger equation and some basic stuff) but I didn't do anything deep about it. I was wondering what should I read to be able to make the transition from Quantum Mechanics to QFT? Thank you!